More requirements on data loading

[emmaai]

Expected behaviour

• Aggregate over observations if large polygons cross landsat path/row such that no single observation can cover majority area (>90%) of polygons.
  Special requirement: Aggregate by the rule of "filling the hole", i.e., only fill the nodata pixels from previous observation with those from later observations.

Actual behaviour

N/A

Steps to reproduce the behaviour

N/A

Environment information

N/A

[uchchwhash]

Also relevant to issue #19.