Required inputs

VaspCalculation:

• parameters - We anyway need to create an INCAR file so requiring parameters makes sense. Also, if required, user need to submit an empty parameter set and then the user know they run with the default tags.

• kpoints - We should always require the kpoints to keep as true to provenance. Users can still use the AiiDA functionality regarding k-spacing etc.

• structure - We need the structure to execute VASP.