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
Introduce vec_basic in Cython

Use it instead of VecBasic
Remove VecBasic
Merge pull request #1 from certik/wrappers-fix

Wrappers improvements