fixing "Using a DataArray object to construct a variable is ambiguous, please extract the data using the .data property" issue introduced in xarray 0.19.0

- example: noj_terrain_following.py
- move all distance calculation to distance.py
- old TerrainFollowingDistance function(from Movflot) renamed to TerrainFollowingDistance2
- new simpler and more efficient TerrainFollowingDistance implemented
- WaspGridSite renamed to WaspGridSiteBase.
- New backward-compatible WaspGridSite that allows different Distance calculation objects
- tests