Tested with Boost 1.53 on CentOS 7.5 so it still works with old Boost.

- Introduced max_mappings_extensive attribute in QSscorer to control limit of
  checks to do in (slow) extensive chain mapping algorithm.
- Added qs-max-mappings-extensive command line flag to compare-structures to
  set QSscorer.max_mappings_extensive
- Clean error handling for failures in chain mapping in compare-structures

Fixed an issue in the mmCIF reader for files starting with a ligand only chain. Before the first atom was stored in its own residue and the remaining atoms in a second residue. Happend for PDB entry 1AGA.