For use of a cleaner Slices object without waiting the merge of new datamodel slices mechanism.

- mapper SAFEMSIL1CGranuleFile : improve viewing angles interpolation by extrapolating original low res grid.

2-d time field for Grid feature now has to be written as two separate variables : netCDF4 constraint

- modify Slices object : new definition of what Slices can accept as inputs (e.g. an int for integer indexing is now allowed).

-- make self._fields default to an empty OrderedDict rather than an empty dict.
-- allow fields argument of __init__ to be an OrderedDict.