ENH: Add in remaining stellar mass quantitiy

[WillHartley]

Discussed in #536

^{Originally posted by WillHartley March 24, 2022}
In the k-correct templates we have the mass in stars formed, and the mass remaining (after mass loss due to stellar winds, SNe etc.).
We're preparing a PR to allow use of stellar mass remaining. Current behaviour is that the function, stellar_mass(), refers to stellar mass formed. IMO, the default should be to return the remaining stellar mass.

Proposal:
We have two functions,
stellar_mass_formed()
stellar_mass_remain()

Then the function,
stellar_mass()
simply calls stellar_mass_remain()

Following the discussion, we cannot change the meaning of stellar_mass(), and so we will need to simply add a stellar_mass_remain() function.