Should the user be able to pass just total (solute + solvent) density, or solvent density also as a parameter to Universe.solvate() ?

**Is your feature request related to a problem? Please describe.**
The solvate method in the Universe class as currently being implemented takes the total density (solute + solvent) as a parameter.

If the user has a universe containing solute molecules and wants to solvate the system with a given density of solvent, they will have to perform some tedious calculations to work out the density that must be passed as a parameter to achieve the desired density of the solvent that surrounds their solute.

**Describe the solution you'd like**
There could be a keyword argument, perhaps a bool, that indicates whether or not the total density or just the solvent density is being passed as a parameter.

**Describe alternatives you've considered**
An extension of this feature could be that passing ``density=None`` solvates the universe with the density of the solvent as it is in its equilibrium configuration (i.e. the density of the box defined in configurations.py).

This would of course only work if a model is specified (i.e. ``model='SPCE'``), and not if the solvent Molecule object was specified in the ``solvent`` keyword.