Introduce Robust PCA.

Add the support of Robust PCA computation in vtkPCAStatistics.
This is performed by computing median instead of mean in the
vtkMultiCorrelativeStatistics class (done when turning
on the MedianAbsoluteVariance flag). Support is also done
in the parallel version of the algorithms.
Add 2 tests for this new robust implementation.

Change-Id: If91813722c7528b96e6594a3b6e21ebcf3e6b455
Merge topic 'IntroduceRobustPCA' into master

5a31f33a Introduce Robust PCA.