Remove _matrix_c2py()
Remove _matrix_c2py()

Refactored `inv()`, `mul_matrix()` and `add_matrix()` methods to stop using
`_matrix_c2py()`

symengine/symengine.py,bjodah/symengine.py,bjodah/symengine.py,symengine/symengine.py,bjodah/symengine.py,symengine/symengine.py