Check for number of atoms in CalculateRMSD

This function assumes the user knows what he's doing, this is also clearly
stated in the documentation:
"RMSD of atom positions (taken blindly in the order in
which the atoms appear) in the two given views."
This check at least avoids a segfault when the number of atoms does not
match. The result would be completely wrong anyway...

modules/mol/alg/src/svd_superpose.cc

@@ -113,6 +113,13 @@ Real CalculateRMSD(const mol::EntityView& ev1,
                    const mol::EntityView& ev2,
                    const geom::Mat4& transformation) {
 
+  if(ev1.GetAtomCount() != ev2.GetAtomCount()) {
+    std::stringstream ss;
+    ss << "Number of atoms in input views must be consistent! ";
+    ss << ev1.GetAtomCount() << " vs. " << ev2.GetAtomCount();
+    throw Error(ss.str());
+  }
+
   return calc_rmsd_for_atom_lists(ev1.GetAtomList(), ev2.GetAtomList(), 
                                   transformation);
 }