Integrate finc/vufind-results-grouping

[mtrojan-ub]

A first attempt to integrate the external module from FINC.

Since the module is made for an old VuFind (+PHP)-Version, some of the original files need to be modified. This cannot be done by including the original module via composer, so i tried to move the code into the VuFind module structure, modify it directly & add parts the code into existing TueFind templates whenever necessary (e.g. result-list.phtml).

TODO

• Use a proper field for grouping (unfortunately the work_keys_str_mv cannot be used => multivalued), maybe we have to create a new field, and generate strings using Java code
• Use Solr's "Collapse and Expand" mechanism instead of old-fashioned "Result Grouping"
• Fix pagination
• Check for more bugs / Side effects
• Check what happens with other Search tabs (especially authority)
• Check conflicts regarding FRBR
• Also enable for other instances (non-ixtheo) + make sure inheritance / hierarchy works properly
• Discuss with community + people from FINC whether it might make sense to migrate this to vufind-org