Major reorganization and tests.

* A new generic Interpolator has been introduced.
* The SatelliteInterpolator is renamed to GeoInterpolator
* The GeoInterpolator uses the generic Interpolator
* SatelliteInterpolator is an alias for GeoInterpolator
* Added regular unittests instead of heavy doctests.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>
Bugfix: new_data attr was not initialized correctly in GeoInterpolator.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>
Update documentation with new interface.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>
Bump up version number to v0.3.0

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>