Parsing the _r file to obtain dipole elements

[thomaslettau]

I have written a little modification that can read wannier90's *_r.dat output and create the transition dipole elements. If this is relevant, I could try to match the style of the project and create a pull request. I might need some help with the proper implementation, since I'am unsure if I should also go for the possibility of sparse matrices and how to deal with iterators properly.