Wrappers for DenseMatrix class

I had to re-declare these functions here. Otherwise, I can't call them from [here](https://github.com/thilinarmtb/csympy/blob/wrappers_II/src/dense_matrix.cpp#L124) and [here](https://github.com/thilinarmtb/csympy/blob/wrappers_II/src/dense_matrix.cpp#L134). Interesting thing here is that the compiler can resolve the call to [submatrix_dense](https://github.com/thilinarmtb/csympy/blob/wrappers_II/src/dense_matrix.cpp#L114) which is not declared twice. The only difference here is that, I use each of these functions inside a member method which have the same name as them but with different method signatures. Seems like member method and a friend function can't have the same name? Can I avoid the re-declaration somehow?