transformed tests to embryonic unit tests and made "value strings" tests pass
better docstrings in utils
* implemented "new syntax": using strings for group of values everywhere
* converted tests to unittests
* when creating an "aggregated" LArray (via .sum(axis=(x, y, z)), use the ValueGroup themselves as the axis ticks, instead of relying on a labeler
* LA.sum(axis=singlegroup) collapse the dimension, like filter
Merge remote-tracking branch 'origin/master'

Conflicts:
	larray.py
	test_la.py