Remove unrecognised variable warning on grid load

[mikekryjak]

This thing prints a warning every time I load a Hermes-3 dataset:

xBOUT/xbout/load.py

Lines 1155 to 1162 in 9b67b37

	if len(unrecognised_dims) > 0:
	# Weird string formatting is a workaround to deal with possible bug in
	# pytest warnings capture - doesn't match strings containing brackets
	warn(
	"Will drop all variables containing the dimensions {} because "
	"they are not recognised".format(str(unrecognised_dims)[1:-1])
	)
	grid = grid.drop_dims(unrecognised_dims)

It's picking up closed_wall_R and closed_wall_Z, which are 1D arrays defined over the time dimension (I guess for a lack of a dummy dimension), and so cause an error due to not being over [x,y,z].

This PR removes the warning. If anyone wants these variables, they can look to modify the code further to make sure they are read properly.

[ZedThree]

See also warnings.filterwarnings to turn this off

[mikekryjak]

It's better to keep the wall coords instead of silently dropping them. Now implemented in #350.