allow loading on Python 3 a dataset created using Python 2
added comment about memory usage
added commented/failing test for boolean indexing
fix AxisCollection on python3
fix on python3
added test to target an array aggregated using string groups by both strings (their def) and a VG with the same values
added read_hdf test (in __main__)
added test for set(value, axis=stringkey)
cleanup .set()

there is no need to call to_key there as it is done in Axis.translate if necessary
better docstrings/comments
cleanup Axis.translate
renamed to_labels to to_ticks
make tuple ticks strings representation the same as list, so that tuple and strings are interchangeable
fix/improve comments
rename str_to_range to slice_str_to_range and add docstring
AxisCollection tests do not use an LArray