bug fix with nested_data
bugfix: wrong coordinate metadata in projections
set masked=True by default in projection
==> bug in unit test: nans get inserted for multi-dimensional projection
bugfix: basemap interp added is_scalar dependency
- bugfix: masked set to np.nan by default (instead of True) ==> py.test
  passes
- bugfix: transform_vectors now works (conserves angle)
- test_crs_vectors : for now simply a routine with plot for visual
  check, apparently a problem remains as far as the magnitude is
concerned, this could be related to cartopy implementation (TODO: check
that)
TODO: see cartopy.trannsform_vectors.vector_scalar_to_grid &
_interpolate_to_grid, see which performs best in term of transform +
regridding