Problem::getState/getTimeStamp considering not initialized processors

The implementation of Problem::getState/getTimeStamp() first implement the case processor_is_motion_list.empty() (taking last KF state or zeroState) and otherwise, fill the state/timeStamp iterating over the processors. But they could be not initialized yet so returning empty state or 0 timestamp (see related issue #333).

The implementation should be in the opposite order: first iterate over processors and afterwards deal with incomplete or missing state/timestamp.

Also, in case of no KF and a prior has been provided by the user, it is better if getState() returns this state than zero.