Added exception and unittest for dist_mat_to_vec of analysis/psa.py (Part of #597)

package/MDAnalysis/analysis/psa.py

@@ -573,6 +573,10 @@ def dist_mat_to_vec(N, i, j):
     :Returns:
       int, index (of the matrix element) in the corresponding distance vector
     """
+    try:
+        validate_dist_mat_to_vec_inputs(N, i, j)
+    except Exception as E:
+        raise E
     if i > N or j > N:
         err_str = "Matrix indices are out of range; i and j must be less than"  \
                 + " N = {0:d}".format(N)
@@ -591,6 +595,30 @@ def dist_mat_to_vec(N, i, j):
         raise ValueError(err_str)
 
 
+def validate_dist_mat_to_vec_inputs(N, i, j):
+    """
+    Validate the inputs of the function dist_mat_to_vec else raise an exception
+    :param N: int
+    :param i: int
+    :param j: int
+    :return: null
+    """
+
+    # Validating if the inputs are integer
+    try:
+        NValue = int(N)
+        iValue = int(i)
+        jValue = int(j)
+    except ValueError:
+        raise ValueError
+
+    # Check if the inputs are not out of bounds
+    if N-1 > i > 0 and N >= 2 and i < j < N:
+        pass
+    else:
+        raise ValueError
+
+
 class Path(object):
     """Pre-process a :class:`MDAnalysis.Universe` object: (1) fit the
     trajectory to a reference structure, (2) convert fitted time series to a