[yt-dev] RFC: introduce {length, mass}_units (similar to time_units)

Hi, I've been hit by a part of the code (plot_modifications:get_smallest_appropriate_unit) that assumed my data knows what a Mpc, or even Rsun is. While it's trivial to workaround by using try/except, that made wonder why time_units get a special treatment in StaticOutput? Would it be beneficial to introduce corresponding {lenght,mass}_units? Cheers, Kacper