The mls_results_ vector did not need to be allocated for most of the
Moving Least Squares methods. However, a reference to an entry in this
vector was needed for calls to computeMLSPointNormal, which is why it
was being allocated in full. This commit causes the vector to be resized
to 1 for those methods that don't need the full version, and then a
reference to entry 0 is passed. The decision was made not to modify
computeMLSPointNormal because it needs to be thread-safe and I do not
have the means to test the OpenMP version of the code. Identical
modifications were made to MovingLeastSquaresOMP::performProcessing as
to MovingLeastSquares::performProcessing.

Tests which do not depend on OpenMP still pass.
Merge pull request #785 from mohawkjohn/eliminate_mls_result_resize

Reduced space usage for Moving Least Squares