Generalize "elements" that can be tracked to include other quantities (e.g. number of SNe)

[abensonca]

Currently, the stellarPopulationProperties class (and related code) allows tracking of different elements as they are produced by star formation. This could be generalized to allow tracking of any product of star formation, e.g. Type Ia SNe.

This would require generalizing/modifying the abundances class to allow it to handle non-abundances. Or, probably better (since abundances are often needed for other things), just create a new class, e.g. stellarRemnants, to track these quantities.