heuristics for parameter recommendations

[jkitchin]

a static function to recommend parameter settings:

encut, kpts, ismear, nbands, sigma

based on the geometry and composition

This function would scan POTCARs for encut recommendations, try to determine if you are a molecule/metal/semiconductor, etc... store information about kpt densities needed, maybe analyze symmetry to determine if the k-grid should be shifted, etc...