allow user defined models to set there own html markup on units

[pkienzle]

Currently if you want to do something fancy like $\Ohm^{-1}$ for the units in the documentation you need to modify generate.py and add an entry to the RST_UNITS map, converting the GUI form name 1/ohm that is stored in the parameter definition to the appropriate markup in html file.

We can extend the unit definition field so that it contains "units|markup", and parse it into two different properties when the model is loaded, one for the GUI and one for the documentation.

This only matters for third-party models; for now we can update generate.py as needed.