Localization Mode + MM: Add option to pre-optimize the full global graph once so that map origin doesn't change on retrieval

[matlabbe]

See original discussion here: http://official-rtab-map-forum.206.s1.nabble.com/MemoryThr-td9766.html

I think I understand what you are implying. It could be indeed a nice option if we could save the full WM+LTM optimized graph in database (or do single global map optimization on start with all nodes in WM and LTM when launching in localization mode) so that the root is always the oldest node in the database. On retrieval, we don't need to optimize anything anymore that would make the map origin to change, we just take the pose from the WM+LTM optimized graph. The poses will be always in same map frame even if memory management is enabled (so we can use RGBD/OptimizeFromGraphEnd=false).

Related to #1201