[Bug] Support negative and string indices for DataFrame

[thetorpedodog]

Accessing a negative index of a dataframe currently causes an error. Per single-cell-data/SOMA#121 (comment) it should not (only when the domain is non-negative).

TileDB-SOMA/apis/python/src/tiledbsoma/dataframe.py

Lines 201 to 204 in d1feac9

	if lo < 0 or hi < 0:
	raise ValueError(
	f"slice start and stop may not be negative; got ({lo}, {hi})"
	)