Current array indexing assumes low integer IDs

[cmutel]

This breaks use of Snowflake IDs.

See comment at

matrix_utils/matrix_utils/array_mapper.py

Lines 32 to 37 in 087757b

	# TODO
	# Sparse matrices could be used if the number of values present is much less
	# than the number of possible values, given the (min, max) range.
	# The default code will generate a complete mapping for the (min, max)
	# interval, which can use too much memory in certain cases.
	# self.use_sparse = len(self.keys) / self.keys.max() <= sparse_cutoff: