Refactor and Fix compiler warning

* Rename `push` to `push_back` in VecBasic class
* Fix the compiler warning due to the missiing distructor in MatrixBase class
Remove VecBasic API from DenseMatrix

Remove the VecBasic interface from DenseMatrix. The VecBasic is there for
Python wrappers only and it should not pullute the C++ interface.

Implement VecBasic::get_vec() and use it in the Python wrappers to pass the
vec_basic directly.
Improve comment to VecBasic