Allow setting up k-points from k-point density descriptor

[awvwgk]

Is your feature request related to a problem? Please describe.

Setting up several DFTB+ inputs in an automated way requires knowledge about the k-point grid. Using one k-point grid (like MP 2x2x2) for all input might be suboptimal for some inputs

Describe the solution you'd like

Specify a k-point density and have DFTB+ generate the corresponding MP grid for the respective cell.

Describe alternatives you've considered

Extend my input generator to figure out the right k-point grid from the cell information.

[aradi]

Very good idea! Maybe, we should couple it with the possibility of providing simplified input for the Monkhorst-Pack scheme:

KPointsAndWeights = MonkhorstPack {
  GridDensity [lenghtUnit]  = d1 d2 d3  # Density in along the 3 reciprocal space vectors
  GridDensity [lengthUnit] = d            # alternative form, if unique density is needed in all 3 directions
  GridPoints =  n1 n2 n3                     # Exclusive with GridDensity for simplified input for MP-scheme
  GammaCentered = Yes/No
}