Gap and gapfill cleanup

[vergauwenthomas]

The gap and gapfill modules could use a code cleanup! Here are some suggestions:

• [ ] Make a distinction between obstype and obstypename. I think it is better to pass Obstypes around in the gap module (not in gapfill module)
• [ ] Do not set gap attributes in the gapfill module !! Rather pass them back to the gap module and set them there.
• [ ] Check which attributes are obsolete, and try to minimize them.
• [ ] Use setter functions for gapfill, so the same data structure is used for different fill methods.