Rename Species mass field

[DavidSagan]

One mistake I can see people making is to use species.mass instead of mass(species) and then wondering what went wrong. To help avoid this, I propose modifying the mass field of the Species struct. So instead of species.mass use the name species.mass_in_std_units (or any other name that is descriptive).

[rot4te]

I renamed the struct field in the most recent PR for branch '75+-edit_species_mass_unit', but I haven't renamed the same field in either the atomic or subatomic species dictionaries. Should I?

[DavidSagan]

@rot4te Yes a rename in atomic and subatomic species dicts would make things clearer.

[DavidSagan]

Thinking about it, a rename of the other fields of Species would make things clearer.