Separate Structural Dry Mass and Residual Wet Mass in DynamicMassBody #101

The dynamic mass model (https://github.com/nasa/cml/tree/main/models/dynamics/mass/dynamic_mass) could be enhanced to better support higher‑fidelity simulations in models that depend on it, and this issue is intended as a feature suggestion.

Currently, DynamicMassBody exposes a residual_mass value, defined as the constant baseline mass always present in the body. This value is used together with consumable_mass to compute the total mass. However, residual_mass lumps together two conceptually distinct quantities:

• The structural body dry mass (e.g. tank shell)
• Any residual (non‑consumable) wet mass that remains in the body after depletion (e.g. trapped propellant, unusable ullage)

Some downstream models, such as slosh or other fluid‑dynamics models, may need access specifically to the residual propellant mass and should not include the structural dry mass of the body itself. Right now, both contributions are merged into residual_mass, so this distinction cannot be made.

To support this, the model should separate these quantities by introducing distinct values for structural dry mass and the non‑consumable residual mass, and compute total mass as the sum of dry mass, residual mass, and remaining consumable mass.