This reverts merge request !98

- Removed xarray

See merge request es/NES!98

Added some chenges that comes from the raster implementation but there are not related with the raster. Those are code improvements as removing xarray dependence.

# Conflicts:
#	nes/nc_projections/default_nes.py
#	requirements.txt

sum_axis: fixed issue where level was a list instead a numpy array

See merge request es/NES!97

Develop 74 vertical interpolation extrapolation

See merge request es/NES!96

# Conflicts:
#	CHANGELOG.md

- Starting open_raster()