fix issues working with code units

[ngoldbaum]

This fixes some issues I found trying to get unyt working with yt. First, it adds an explicit test for a code unit system like yt's. It also makes sure that custom units will show up in a unit container object like i've created in yt via ds.units, to make sure that things like ds.units.code_length will work as expected. I also added a new has_current_mks property on the UnitSystem class that I found useful in the process of getting yt working with unyt.