Fix concat_<side>_guards coords assignment

[mikekryjak]

Fixes #331.

In short, _concat_upper_guards and its 3 friends for lower, outer and inner guards had coord assignments of this type:

xBOUT/xbout/region.py

Lines 1345 to 1353 in 9c634a4

	# can't use commented out version, uncommented one works around xarray bug
	# removing attrs
	# https://github.com/pydata/xarray/issues/4415
	# https://github.com/pydata/xarray/issues/4393
	# da_inner = da_inner.assign_coords(**{xcoord: new_xcoord, ycoord: new_ycoord})
	da_inner[xcoord].data[...] = new_xcoord.data
	da_inner = da_inner.reset_index(xcoord).set_xindex(xcoord)
	da_inner[ycoord].data[...] = new_ycoord.data
	da_inner = da_inner.reset_index(ycoord).set_xindex(ycoord)

According to the comment, this approach was chosen due to a bug. I'm not sure if I've followed the Xarray issue chain properly, but it now seems fixed: pydata/xarray#4393

This approach doesn't work anymore because coordinates are read only by default.... and I am very confused why it worked for me not that long ago! I have replaced it with the proper Xarray assignment.

I based this PR off a commit before #317 because that PR breaks loading any simulations for me (#329). Let's merge this one after #329 is resolved.

[johnomotani]

Just for historical interest: the mentioned issues were closed as duplicates, not fixed. The original issue is still open (pydata/xarray#2180) but has no update since 2018, so guess it won't ever be fixed. This PR looks like a different workaround for the same issue, but that's not a problem as long as it works!