Relax hardcoding of "lon" and "lat" for input data

[VeckoTheGecko]

Looking at

Parcels/src/parcels/_core/xgrid.py

Lines 176 to 183 in 08a0a26

	# Set the coordinates for the dataset (needed to be done explicitly for curvilinear grids)
	if "lon" in ds:
	ds.set_coords("lon")
	if "lat" in ds:
	ds.set_coords("lat")
	if len(set(grid.axes) & {"X", "Y"}) > 0: # Only if spatial grid is >0D (see #2054 for further development)
	assert_valid_lat_lon(ds["lat"], ds["lon"], grid.axes)

I think there is some degree to which we expect the F points to be named "lat" and "lon" in the input data. We should remove this assumption, instead referencing node_coordinates in the SGRID metadata.