[WIP] Add support for xarray with dask data to satpy

[mraspaud]

As the title says. It will probably break a bunch of stuff though.

• Closes NetCDF writer doesn't work #123
• Tests added
• Tests passed
• Passes git diff origin/develop **/*py | flake8 --diff (remove if you did not edit any Python files)
• Fully documented
• Reuse same kdtree instance for channels with same mask
• Make data reducing work for resampling

[coveralls]

Coverage decreased (-2.6%) to 57.896% when pulling 4c7b8ba on feature-xarray into ee8934a on develop.

[coveralls]

Coverage decreased (-2.7%) to 57.823% when pulling 24da07e on feature-xarray into ee8934a on develop.

[coveralls]

Coverage decreased (-2.6%) to 57.878% when pulling 7cdddf0 on feature-xarray into ee8934a on develop.

[coveralls]

Coverage decreased (-2.6%) to 57.882% when pulling f211ceb on feature-xarray into ee8934a on develop.

[coveralls]

Coverage decreased (-2.5%) to 58.063% when pulling a7e93fd on feature-xarray into ee8934a on develop.

[coveralls]

Coverage decreased (-2.5%) to 58.063% when pulling a7e93fd on feature-xarray into ee8934a on develop.

[coveralls]

Coverage decreased (-2.8%) to 57.751% when pulling a7e93fd on feature-xarray into ee8934a on develop.

[coveralls]

Coverage decreased (-2.8%) to 57.677% when pulling dee8fcb on feature-xarray into 06248d5 on develop.