Implement `__sympy__()` method and `densematrix()` function
Add tests for `get` method and fix a typo

* Add tests for `get()` method
* Use `_sympy_` instead of `__sympy__`
* Raise an exception if matrix type is not `DenseMatrix`.
Add tests for sympy conversion and `set()` method
Add tests for sympy to csympy conversions
Fix a bug in __richcmp__ method for Matrix classes