Implement efficient DOS calculation

[MRedies]

Right now the Eigenenergies and Eigenvectors for the whole k-space are calculated for every E-point in the DOS calculation. This is very slow and can be improved, if the nesting is switched:

for k-points:
     calc Ham(K)
     for E points:
             do stuff

rather than

for E-points:
     for k-points:
          calc Ham(k)
          do stuff

[MRedies]

did that