Merging of two or more CrystalMap's with different phases into one CrystalMap

[hakonanes]

We have this functionality in kikuchipy: https://github.com/pyxem/kikuchipy/blob/master/kikuchipy/indexing/_merge_crystal_maps.py. I hope we can move this to a orix.crystal_map.merge() function (not CrystalMap method) for v0.7.