sorter: multiple tag sorting fixes
- Fixes issue in sorting according to multiple MARC tags.
- Fixes issue with ordering of sorted records as well as handling of unsorted records.
- Fixes record slicing issue using 1-indexed value instead of 0-indexed value.
- Adds generic method to decide whether to rank or sort records and changes facets to use this method.
- Adds two configuration variables to control default sort field and order.
Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>