MRG was originally distributed by mulgara.org (it was the RDF parser used by the Mulgara database), but for some reason or another it's no longer maintained (or not maintained separately). mrg-0.7 is still available in the Maven repository, and presumably won't disappear, but it isn't readily available in source form.
It would be at least tidy to migrate to another RDF parser, and (by using a library more easily available) would make it easier for Unity to be packaged for eg Debian.
Possibilities include:
(JRDF is inactive, and sesame / OpenRDF.org has vanished).
MRG is currently used only in src/buildtools/src/uk/me/nxg/unity/ParseUnits.java so swapping it out shouldn't be a massive challenge.
I'm using RDF4j. It mostly works, except... (inevitably) the graphs are queried in a different order in different libraries, and thus I discover that the previous version worked slightly by accident.
Damn: I have managed to make this long in my career without ever having to make sense of CGS-ESU units and their somewhat unexpected dimensions.
Closed with commit https://hg.sr.ht/~nxg/unity/rev/8c192b6d458d1d3936fce9561660c95c27636ab8