ENH: vasp/create_input.py can find fallback POTCAR when setup is not given or wrong.

This enhancement allows for the VASP calculator initializer to find reasonable pseudo-potential files when the setup is entered incorrectly, does not exist or is not specified.

For instance 'Ca' has only '_pv' and '_sv' setups, no single 'Ca' flavour. Then when not specifying the setups manually as e.g. {'Ca':'_pv'}, the vasp calculator fails starting with e.g. error 'No pseudopotential for Ca!'

In such case, this modification will try in order e.g. 'h','pv','sv', 's', 'd', ... versions of the POTCAR directories, i.e. build automatically a 'setups' choice which corresponds to what is actually available in the 'potpaw' directories.