Merge pull request #6097 from jreback/hdf_fix

TST: don't compare inferred index freq on some tests as it may not be preserved (which is correct)
BUG: not converting scalars properly to M8/m8 on assignment (GH6079)
CLN: removed need for SNDArray ; simplied SeriesGroupber / SeriesBinGrouper
TST: skip read_dta2 on non-little endian
BUG: rework object inferernce with NaN as the first element in an array
BUG: convert datetime like better from list-of-lists
BUG: make rank of M8 work via object algos