Add missing Voloctree Mapper rank-overlapped ids getter

[marcocisternino]

This branch:

• aligns rank-overlapped reference ids getter (getSentReferenceIds) to the mapped ids ones , by providing non-empty rank lists only
• changes the return datatype of these getters into an unordered_map
• introduces a missing getter, specifically the getReceivedReferenceIds method

It has been tested using rank/ids list getters for mesh mapping MPI communications in an application code.

[andrea-iob]

To be consistent with current git history, the portion of the commit title should only contain the name of the module (e.g., "voloctree: ...").