Prevent user from declaring variables and functions with same name

* Today use can "declarae" a variable in NEURON block and
  then PROCEDURE or FUNCTION with the same name. For example,
  below doesn't raise any error today:

   ```python
   NEURON {
       SUFFIX glia
       RANGE alfa
   }

   FUNCTION alfa(v(mV)) {
       alfa = exp(v)
   }
   ```

* The reason this works is that the `RANGE alfa` declaration in
  NEURON block id not complete (i.e. variable needs to be "defined"
  in block like ASSIGNED, PARAMETER etc) and hence doesn't appear
  anywhere in the generated code. If we try to use `alfa` as variable
  then that is warning and resulted into compilation error.

* This behaviour kind of works but error prone. Apart from corner cases
  and confusing behaviour, it's difficult to do certain analysis that
  we would like to do (e.g. in new NMODL code generation).

* So for the robustness and simplicity, we now throw an error if user
  try to declarte variable and function with the same name.

  ```console
   $ ./bin/nocmodl mod_x/test.mod
       Translating mod_x/test.mod into mod_x/test.cpp
       Error: alfa used as both variable and function in file mod/test.mod
  ```

src/nmodl/consist.cpp

@@ -19,6 +19,30 @@ extern Symbol* indepsym;
             err = 1;                                        \
         }
 
+/**
+ * Check if variable declared in NEURON block is conflicting with a PROCEDURE or FUNCTION
+ *
+ * In order to avoid "declaring" variable in NEURON block and then defining
+ * PROCEDURE or FUNCTION with the same name, we check if the variable is
+ * function and has one of the NRN* type.
+ */
+int is_var_declared_as_function(Symbol* s) {
+    int type = s->nrntype;
+    int usage = s->usage;
+
+    // if not function or procedure name then return already
+    if (! (usage & FUNCT) ) {
+        return 0;
+    }
+
+    // conflicting name if presence of NEURON block variable type
+    return ( type & NRNRANGE
+            || type & NRNGLOBAL
+            || type & NRNPOINTER
+            || type & NRNBBCOREPOINTER
+            || type & NRNEXTRN);
+}
+
 void consistency() {
     Symbol* s;
     Item* qs;
@@ -51,10 +75,18 @@ void consistency() {
         con("STEPPED", STEP1, 0);
         con("CONSTANT UNITS FACTOR", UNITDEF, 0);
         tu = s->usage;
-        if ((tu & DEP) && (tu & FUNCT))
+        if ((tu & DEP) && (tu & FUNCT)) {
             diag(s->name, " used as both variable and function");
-        if ((t == 0) && tu)
+        }
+        // check for conflicting variable declaration with function
+        // do not use diag() because line number might be misleading
+        if (is_var_declared_as_function(s)) {
+            Fprintf(stderr, "Error: %s used as both variable and function in file %s\n", s->name, finname);
+            exit(1);
+        }
+        if ((t == 0) && tu) {
             Fprintf(stderr, "Warning: %s undefined. (declared within VERBATIM?)\n", s->name);
+        }
     }
     if (err) {
         diag("multiple uses for same variable", (char*) 0);

src/nmodl/io.cpp

@@ -233,7 +233,7 @@ char* current_line() { /* assumes we actually want the previous line */
 /* two arguments so we can pass a name to construct an error message. */
 void diag(char* s1, char* s2) {
     char* cp;
-    Fprintf(stderr, "%s", s1);
+    Fprintf(stderr, "Error: %s", s1);
     if (s2) {
         Fprintf(stderr, "%s", s2);
     }

src/nmodl/modl.h

@@ -215,6 +215,24 @@ typedef struct Symbol {
 #define EXTDEF5       040000000L /* not threadsafe from the extdef list */
 #define EXPLICIT_DECL 01         /* usage field, variable occurs in input file */
 
+
+#define NRNEXTRN     01          /* t, dt, celsius, etc. */
+#define NRNCURIN     02          /* input value used */
+#define NRNCUROUT    04          /* added to output value */
+#define NRNRANGE     010
+#define NRNPRANGEIN  020
+#define NRNPRANGEOUT 040
+#define NRNGLOBAL    0100        /* same for all sections, defined here */
+#define NRNSTATIC    0200        /* v */
+#define NRNNOTP      0400        /* doesn't belong in p array */
+#define NRNIONFLAG   01000       /* temporary flag to allow READ and WRITE \
+			                        without declaring twice */
+#define NRNSECTION       02000
+#define NRNPOINTER       04000
+#define IONCONC          010000
+#define NRNBBCOREPOINTER 020000
+
+
 extern char *emalloc(unsigned), /* malloc with out of space checking */
     *stralloc(char*, char*),    /* copies string to new space */
     *inputline(),               /* used only by parser to get title line */

src/nmodl/nocpout.cpp

@@ -85,22 +85,6 @@ not thread safe and _p and _ppvar are static.
 */
 
 #endif
-#define NRNEXTRN     01 /* t, dt, celsius, etc. */
-#define NRNCURIN     02 /* input value used */
-#define NRNCUROUT    04 /* added to output value */
-#define NRNRANGE     010
-#define NRNPRANGEIN  020
-#define NRNPRANGEOUT 040
-#define NRNGLOBAL    0100 /* same for all sections, defined here */
-#define NRNSTATIC    0200 /* v */
-#define NRNNOTP      0400 /* doesn't belong in p array */
-#define NRNIONFLAG                                  \
-    01000 /* temporary flag to allow READ and WRITE \
- without declaring twice */
-#define NRNSECTION       02000
-#define NRNPOINTER       04000
-#define IONCONC          010000
-#define NRNBBCOREPOINTER 020000
 
 #define IONEREV 0 /* Parameter */
 #define IONIN   1