asset_per_band: improve partitioner

[jdries]

The final stage of asset_per_band jobs has bad partitioning, leading to a large amount of useless tasks.
The fix is to change to a different partitioner when doing 'groupbykey'. A good partitioner for this kind of operation, is probably assigning one key to one partition.
Here's a partitioner that does exactly that, based on an array of keys:

openeo-geotrellis-extensions/geotrellis-common/src/main/scala/org/openeo/geotrelliscommon/package.scala

Line 24 in 64d4812

class ByKeyPartitioner[K](splits: Array[K]) extends Partitioner {