Include statistics for `solvents` that aren't participating in solvation

[orionarcher]

Currently, if a molecule is included as a solvent but isn't in any solvation shells, it will not be included in various dataframes and dictionaries that tabulate results. This leads to an unpredictable API where I cannot be certain if retrieving a value will work until I know the results.

e.g.

solute =  #  create solute
solute.run()
solute.coordination.coordination_numbers["A_SOLVENT"]  # will this error or not?