provide reference for constants used

[rhaas80]

The UnitConversion docs https://github.com/ChunHuangPhy/CompactObject/blob/main/docs/source/UnitConventionForDeveloper.ipynb do not give a reverence for the source of the values of the numbers used. In particular for the less constrained values of G and Msun this would be useful, but also in principle for nuclear quantities like the nuclear saturation density or (free?) neutron mass.

[zhoutz]

The source of these values was copied from Wikipedia.
In the following PR, I have included reference links as comments.
#125