Problem local_reference_

In some cases, it is necessary to define a relationship of the problem (local) reference frame and a global one. This may be the case of problems with GNSS or IMU measurements.

Agreed with @jsola: We will add in problem an attribute Problem::local_reference_ (with a type something like struct Reference{ StatePoint3dPtr p_ptr; StateQuaternionPtr o_ptr;).

This local_reference_ could be optionally defined in the YAML by the user or automatically defined in other cases (GNSS for instance).