Adding a `xmap.plot("color")` method

[argerlt]

I think the Phase class should have some sort of method for creating colormaps within the class.

currently, we do this (just like MTEX does)

import orix.data as oda
import orix.plot as opl
import orix.vector as ove

xmap = oda.sdss_ferrite_austenite(allow_download=True)
ckey_m3m = opl.IPFColorKeyTSL(
    xmap.phases["austenite"].point_group, direction=ove.Vector3d.zvector()
)
rgb_au = ckey_m3m.orientation2color(xmap["austenite"].orientations)
rgb_fe = ckey_m3m.orientation2color(xmap["ferrite"].orientations)
xmap['ferrite'].plot(rgb_fe)

Which works well enough, but being able to just directly do

xmap = oda.sdss_ferrite_austenite(allow_download=True)
xmap.plot('tsl')

would (I think) be better.

Open question on if it should be an addition to Phase, or a function in orix.plot.CrystalMapPlot. Former seems more useful, since it can be used for simplifying IPF plots as well, but the latter is maybe better from a "dividing up modules" point of view.