Handle the case when atoms are on top of each other

Do the same as NACCESS: assign 0.0

modules/mol/alg/src/accessibility.cc

@@ -203,6 +203,11 @@ Real GetAtomAccessibilityNACCESS(Real x_pos, Real y_pos, Real z_pos,
     Real a = x_pos - x[i];
     Real b = y_pos - y[i];  
     Real c = a*a + b*b;
+
+    if(c == Real(0.0)) {
+      return 0.0;
+    }
+
     dx[i] = a;
     dy[i] = b;
     dsqr[i] = c;