Document inter-unit-product option
Set up the unit parser

Defining the parser requires that at least one option is added
(`sticky-per`) and also one more piece of scratch data.
Change a few "units" to "siunitx_units"

While the long term plan might be to make the code into totally
separate modules, that's not the case at present.
Add the text for the messages needed thus far

Presumably as this is a submodule the messages should be divided up in
the same way as keys, _etc._