Use the matplotlib units support to deal with conversions in and out of degrees

[Cadair]

It's rather annoying to have to manually convert everything into degrees when plotting on top on an image in arcseconds. It would be cool if we could make mpl unit aware for stuff like this.

[astrofrog]

See also astropy/astropy#3980

Need to check if we can make use of this

[Cadair]

@astrofrog That's what I was thinking of

[astrofrog]

Note from @mdboom

... you can look to the transform and determine what to convert the inputs to in the default_units callback, e.g.:

    def default_units(x, axis):
        if hasattr(axis.axes.get_transform(), 'expected_unit'):
            return axis.axes.get_transform().expected_unit
        ....
This is by design -- we don't want to be doing unit conversion deep in the matplotlib redraw loop.

[Cadair]

I don't really understand that, but it seems possible then?

[astrofrog]

Not clear yet, but that was a note to self :)