Thread safety cleanup in KalmanAlignmentAlgorithm

Make the return value of two const functions be
a const pointer instead of just a pointer.
Eliminates a potential thread safety problem
reported by the clang static code analyzer.
Merge pull request #3313 from wddgit/threadSafetyKalmanAlignmentAlgorithm

Alignment/KalmanAlignmentAlgorithm -- Thread safety cleanup in KalmanAlignmentAlgorithm