bring ncio back in the game
added skipna parameter to since, between and time_mean
fixed bug in GroupedAxis (axes was not excluded from metadata)
slice(None) is detected
ncio feature commented until a strange bug related to netCDF4 is fixed (Dumped Core)
(probably some file not properly closed)

Revert "bring ncio back in the game"

This reverts commit c3f36bc5c01cfb5b74ba7d78c4b6b3fe151f0741.
renamed README to README.rst