Label-based indexing for NXlog with sublogs

[SimonHeybrock]

This is currently not supported, see

scippnexus/src/scippnexus/nxdata.py

Lines 493 to 499 in e33f2f4

	# Sublogs have distinct time axes (with a different length). Must disable
	# positional indexing.
	if self._sublogs and ('time' in _to_canonical_select(list(self.sizes), sel)):
	raise sc.DimensionError(
	"Cannot positionally select time since there are multiple "
	"time fields. Label-based selection is not supported yet."
	)

. As all ESS files seem to have such sublogs we will either be stuck with loading the entire log, or maybe provide a custom application definition when loading NXlog to drop the offending sublogs. Or the mechanism needs to be added, but this may be complicated?

See also #232.