Implement relative altitude filter

[thorbjoernl]

Summary

Implement a relative altitude filter which filters stations based on the relative difference between station altitude and model altitude.

Related Issue

closes #39

[heikoklein]

The specification was a bit unclear. We want to exclude stations which are to far off the gridded topography. To far off is absolute, e.g. 500m independed of topography being at 0m or at 2000m.

[thorbjoernl]

CI fails with error if I don't include netcdf4 as an explicit dependency so I've added it back in for now:

[heikoklein]

Looks very good. Please make dependencies optional (even if the break the altitude-filter if not installed).
Please use some time on performance-tuning.
The case for stations outside the grid is not documented/tested.

[heikoklein]

Very nice. Thanks.