Add easy way to create new substance for specific number of molecules

[lilyminium]

A Substance may be created with mole fractions of 0.5098 for one component and 0.4902 for another, but when actually creating a box of 100 molecules, this gets turned into actual mole fractions of 0.51 and 0.49 respectively. This will pose an issue when querying storage backends for existing equilibrated boxes, as what's in the storage backend (i.e. the latter, simpler mole fractions) won't match the exact substance in the query. Instead, there should be an easy way to create a new, simpler substance matching the total number of molecules for the query.