BibIndex: new abstract class for indexes
- Introduces new abstract class for inheriting for virtual and regular indexes.
- Cleans WordTable argument list.
- Removes unused imports from bibindex_engine file.
Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>