Unit

This is the new version of the previous preprocessing pull request.  Differences are that only necessary files are added (that is, units related), and a much more thoroughfully unit testing and checking is done.

Also, R_universal<>() is in SI, the derivatives, as _P0_R and the scale factor in rate constants are thus corrected.

Have fun reviewing this :)
What are you manually stating the copy constructor here? I don't see anything special such that we can't use the default copy constructor...