Include OptaDos calculation

It is quite awkward to manually export the calculations, write OptaDOS inputs files and project the density of states, when other steps are automated. It would be nice to implement an OptadosCalculation which can be used in CastepSpectralWorkChain (also needs to be implemented), so that DOS are projected automatically.

Care should be taken to design how it is connected with the underlying CASTEP calculation on the provenance graph.