Added a new property, Valid, to BasisSet.
Added a new property, Valid, to BasisSet.

This allows readers to indicate if there was a problem reading an input
file, and mark a basis set as invalid.

OpenChemistry/openqube,OpenChemistry/openqube,cryos/openqube,OpenChemistry/openqube,OpenChemistry/openqube,cryos/openqube