Introduce units corresponding to AB and ST flux density zeropoints

The discussion on magnitudes in #1577 made me realise we should have the flux densities for which the AB and ST magnitudes are 0 as predefined units.  They are introduces with the PR.

Note that I considered whether they should be under astrophysical constants instead, but ended up deciding against it, as I think the way one should think of it is "magnitudes for fluxes in units of..." rather than "magnitudes for fluxes normalised by ...", ie., just like `logg = log(g.cgs)`, so `STmag=-2.5*log(f.to(STflux))`, and what one has is properly a functional unit, not a dimensional quantity.