Merge into EXT:solr

[irnnr]

I think it would make sense to simply move this code into the main EXT:solr extension. The solrgrouping extension is relatively small and the only reason it is separate is that was initially developed as a client project and was kept as an add on for purchase.

Merging the two extensions would reduce effort when updating/changing/refactoring the main extension.

[dkd-dobberkau]

I would be not in favor of this. My opionion is that we should not let the extensions to much grow.