Distinguish between M_planet and M_interior

[nichollsh]

Throughout the code we currently refer to the variable M_planet. However, in some cases this is used as the total planet mass, and in other cases as the mass of the interior (i.e. everything enclosed by the atmosphere-mantle interface).

We should use two variables to describe this: M_planet and M_int, where M_planet = M_int + M_atm, where M_int is the mass of the interior and M_atm is the mass of the atmosphere.

This is connected to #212, #50, #62, #82.

[timlichtenberg]

Good point, and while doing this, we may want to separate it from M_core as well.

[nichollsh]

Yeah that makes sense too. I'll make a PR for this once the current one is merged.

[nichollsh]

We will need to think about how to handle volatile mass partitioning. Should the mass of the interior decrease with outgassing? Do we provide SPIDER/ARAGOG with the dry or wet mantle mass? Or do we assume that volatile do not contribute to M_int? We could leave this for a next step.

If we do allow M_int to decrease with outgassing, then how does SPIDER handle that? Handling this properly will require a structure model, I think.