Only add scans to database if explicitly requested

[Omega3395]

It would be interesting to add an option to insert scans into the database only when explicitly requested (or, inversely, avoid adding scans to the database when explicitly requested).
This feature would be particularly useful when the algorithm is used for re-localization, where one may prefer to only include the map scans in the database (which have been optimized through SLAM and thus represent “clean” poses) but not runtime poses.

Thank you for open sourcing such a valuable project!
Alberto.