Implement band density/STM post-processing with MSSF

[davidbowler]

The post-processing utility does not have the capability to deal with MSSF (and this will require extra output from Conquest as well) for band density and STM simulation.

[davidbowler]

The branch f-band-density-MSSF has a first implementation of band densities generated from MSSF calculations (though not STM calculations). It also allows plotting of MSSF from specific atoms as cube files. The STM (simple TH) implementation will follow soon.

[ayakon]

When PDOS with MSSFs was implemented, the band density and STM with MSSFs were also implemented in the f-pDOS-MSSFs branch. The f-pDOS-MSSFs branch was merged to the develop branch and so closed already.

[davidbowler]

The f-pDOS-MSSF branch pull request is #403