Use Eigen instead of Meschach

.gitmodules

@@ -22,3 +22,6 @@
 [submodule "external/CLI11"]
 	path = external/CLI11
 	url = https://github.com/CLIUtils/CLI11.git
+[submodule "external/eigen"]
+	path = external/eigen
+	url = https://gitlab.com/libeigen/eigen

CMakeLists.txt

@@ -245,6 +245,8 @@ if(NOT EXISTS "${CODING_CONV_CMAKE}/3rdparty.cmake")
 endif()
 include("${CODING_CONV_CMAKE}/3rdparty.cmake")
 cpp_cc_git_submodule(Random123)
+option(EIGEN_BUILD_TESTING "" OFF)
+cpp_cc_git_submodule(eigen BUILD PACKAGE Eigen3::Eigen REQUIRED)
 
 # =================================================================================================
 # Enable sanitizer support if the NRN_SANITIZERS variable is set. Comes befores PythonHelper.cmake.

src/ivoc/matrix.cpp

@@ -16,17 +16,6 @@ extern int hoc_return_type_code;
 extern double hoc_scan(FILE*);
 extern Object** hoc_temp_objptr(Object*);
 
-#if 0
-	extern void install_matrix_method(const char* name, double (*)(...));
-	extern void* matrix_arg(int);
-	extern double* matrix_pelm(void*, int i, int j);
-	extern int matrix_nrow(void*);
-	extern int matrix_ncol(void*);
-	extern int matrix_type(void*);	// FULL 1, SPARSE 2, BAND 3
-	extern MAT* matrix_full(void*); // hoc_execerror if void* not right type
-	extern SPMAT* matrix_sparse(void*);
-#endif
-
 static void check_domain(int i, int j) {
     if (i > j || i < 0) {
         auto const tmp = "index=" + std::to_string(i) + "  max_index=" + std::to_string(j) + "\n";