cdgen: Ensure that bounding box and reference netcdf are compatible

[cwerner]

Check and ensure that a given bounding box and/ or a reference netcdf file align and the extent is clipped or enlarged accordingly.

Possible measures:

• take existing lat/ lon coords of reference netcdf and extend them to the specified bounding box (enlarge grid)
• clip the netcdf file to bounding box if bounding box is contained in netcdf file extent