Check microscopic bimolecular reactions at subvolume level instead of region level

[adamjgnoel]

A potential bimolecular reactant is compared with every other potential reactant in current region as well as neighboring regions where the reaction might take place. This could be computationally very inefficient for large systems. It could be better to maintain lists of what subvolumes the reactants of a particular reactant could be in, as well as lists of what molecules are in those subvolumes. This would take a lot more memory and time to initialize but potentially much faster to execute.