More `PdosWorkChain` flexibility

[zooks97]

Currently, the PdosWorkChain always runs an nscf calculation and both a DosCalculation and a ProjwfcCalculation.

For my use case, I'd like to be able to skip the nscf (i.e. proceed directly from an scf calculation) and choose one or both of the DoS and PDoS calculations to run.

I'm thinking to

• Make all of the exposed inputs optional
• Check in input validation that at least
  • scf xor nscf parent folder are provided
  • dos or projwfc are provided
• Run only the requested calculations
  • parallel cannot be done if only one of dos / projwfc is provided

Does this seem reasonable? I'm starting to think that the PdosWorkChain is trying to do too much. Perhaps it would be better to split things up?