Additional reactionsystem conversion file comments

[TorkelE]

Adds some additional comments to the reactionsystem_conversion.jl file. I tried to go through the file and add comments on what each set of code was doing, to make it faster to read for those unfamiliar (which before I did this did include me to some extent). I think I managed to understand everything in the end, but you should probably still go through carefully (no hurry at all though, this is very low prior).

[TorkelE]

Had this around for a while, and was meaning to fix merge into master and stuff now, but didn't finish. Will close for now and re upload at some other point when actually finished.