Consider changes to System and Moves argument specification

[rsdefever]

Summary

Some arguments required to create the mc.System and mc.Moves objects are specified as lists. In certain cases, e.g., the box_list and species_list, this works OK. In other cases, e.g., mols_to_add and mols_in_boxes it becomes cumbersome. Should we switch to another method of specifying these inputs, e.g., a dict?

Goal

The arguments required to create the mc.System and mc.Moves object should be easy to read/write/understand without being unnecessarily verbose. This will minimize user errors and frustration when learning/using the package.

Additional details

E.g. for a system with two species and two boxes, the current code is:

mols_to_add = [[0,10],[1,100]]

This specifies to add 0 molecules of species1 to box1, 10 molecules of species2 to box1, 1 molecule of species1 to box2 and 100 molecules of species2 to box2. This information is not easy to process from a quick glance at the code. Even for a simple case of a single box and single species, the user needs to specify a nested list, e.g., mols_to_add = [[10]].

One preliminary suggestion from @rmatsum836 is something like:

mols_to_add = {'box1' : [0,10],  'box2' : [1,100]}

This solution trades a small increase in verbosity for an increase in clarity. However, additional questions arise. Can the boxes have any name or do we enforce a standard? Should the species also be named or only the boxes? Even in this example the some reading/writing the code has to associate the first item in the list with the first item in the species list.