Add full potential calculation

[aromanro]

With LMTO (see: https://github.com/aromanro/KKR/issues/1) and DFT, a full potential method can be implemented. The code for DFT can be reused from the DFTAtom project https://github.com/aromanro/DFTAtom.

It won't happen soon, though, I need more motivation and free time for such a thing. Besides, there is a little too much on bandstructure computation compared with other projects, so for a while I'll take care of other subjects.