Add ability to define the number of solvent molecules, rather than just a solvation padding.

[IAlibay]

We currently rely on solvate_topology_nonwater, which has a limited number of parameters.

Ideally we want folks to be able to manually define:

• The number of solvent molecules
• Box vectors
• Mass density (this is an option for solvate_topology_nonwater, it's just not been exposed yet)

This likely would take the form of either calling solvate_topology_nonwater or pack_box directly, depending on whichever parameters are being passed.

It would need more settings validation.