Made sort code more functional for readability.
Merge branch 'master' into hierarchy_cache

Conflicts:
	module/VuFind/src/VuFind/Hierarchy/TreeDataSource/Solr.php
Improved comments; simplified code.
Merge branch 'master' into hierarchy_cache

Conflicts:
	module/VuFind/src/VuFind/Hierarchy/TreeDataSource/Solr.php