Adapt initial (pre_ms) model builder for high mass stars

The approach and approximations of the pre_ms model builder are focused on quite low mass stars. We may want to consider other conceptual approaches that might also do a better job of allowing > 100 Msun stars to be constructed easily.

Start by reviewing the pre_ms model builder and documenting which low-mass-star approximations limit construction of stars above 100 Msun. The work is complete when a concrete conceptual approach for easily constructing those high-mass models is defined and its expected behavior is agreed.

Written by the indexing model from the issue text.