Improve None implementation and fix a few bugs

* the data field is now private
* typing when manipulating None<long> is done right (no implicit
  conversion)
Fix bad gsliced_ndarray to_index computation

* The computation was incorrect when the gslice was partial
* Force the use of the proper assignment operator, even if it seems
  redundant
Merge pull request #145 from serge-sans-paille/fix-gslice

* Improve None implementation and fix a few bugs
* Fix bad gsliced_ndarray to_index computation