Base class for all Dirichlet versions

We originally had one base class that worked as a front end https://github.com/dirichletcal/dirichlet_python/blob/master/dirichletcal/__init__.py#L11

at some point we started ussing the individual classes in dirichletcal.calib

We may need to unify them again into the base class, and add unittests for the base class.