added BaseASRWithUncertainties and BaseMSRWithUncertainties to differentiate between scaling relationships that provide uncertainities from the ones that do not
updated scale rel implementation to extend the proper base classes
check that scaling rel is not abstract class when reporting list of available scaling rels
improved class names, replaced  with
removed get_mag and get_area methods; no longer needed
added utility method to retrieve scaling relationships with sigma
Merge pull request #137 from monellid/point_rup_scale_rel

Create abstract classes to represent scaling relationships with uncertainties