Rename main classes for alignment with openEO concepts

[jdries]

Names in this project are not always aligned with the corresponding openEO concepts, because they date from a time when openEO concepts were not yet so clearly defined.
This is however confusing for developers, making it harder to attract external developers, but also making it harder to understand stack traces.
We also see that concepts used here sometimes leak into support, which is then again confusing for our users.

So we can start building a list for key classes:

FileLayerProvider -> LoadCollectionFromFiles
OpenEOProcessScriptBuilder -> ProcessGraphToSparkJob
package -> SaveResultGeotiff