Units, and the suffering caused by them

[jacob-moore22]

It would be nice, as a usability thing, to have the user explicitly define the unit set being used in the input file. This would also allow for preprocessing checks for consistency, and allow for sanity checks when passing input files around.

[jacob-moore22]

Also, it would be good to dump the input files used to do a simulation into the output directory, for clarity when passing results around.