CLN: replace np.flatten() with np.ravel() if copy is not necessary

BLD: added to vbench, df_to_csv_mixed (mixed frame with mixed dtypes)
Merge pull request #2921 from jreback/ravel

ENH: change flatten to ravel (in internals/pytables)
BUG: formatting for Series printing was multiple Dtype lines
BUG: pathological case on SparseSeries with 0-len array and compression (GH 2931)
Merge pull request #2933 from jreback/pytables_2931

BUG: fixes issue in HDFStore w.r.t. compressed empty sparse series (GH #2931)
Merge pull request #2930 from jreback/series_format

BUG: formatting for Series was printing multiple Dtype lines for long display