Add error & test checks for matrix_operations.py (#925)

* Update matrix_operation.py

1. Adding error checks for integer inputs
2. Adding error checks for matrix operations where size requirements do not match up
3. Added matrix subtraction function
4. included error check so only integer is passed into identity function

* Create test_matrix_operation.py

* Update matrix_ops and Add Test Cases

1. Included error checks in matrix operation.  There were some cases where the functions would not work correctly.

2. PEP8 changes to matrix_operations.py

3. added test cases for matrix operations using pytest.

* Update pytest.ini

Add carriage return to end of file

matrix/tests/pytest.ini

@@ -0,0 +1,3 @@
+[pytest]
+markers =
+    mat_ops: tests for matrix operations