Works with (sigma2 * Id) set in the method as point covariance.
Tests now work for PosteriorModelBuilder using sigma2I as a covariance
matrix, set in a dedicated C++ method.
Now has a C++ test that sets a uniform matrix from the outside.
Removed the bug that the covariance and precision matrix were confused.
The C++ test now has a simple test with a one-point model, but
non-uniform point covariance matrix. Seems to be working.
Posterior model builder now has a test that acutally tests the 3D
posterior with an anisotropic noise model for one point.
Added ITK posterior model builder.
Changed test posterior model builder C++ test to use eigen functionality
rather than VTK for finding tangential vectors.
One typo, one additional function wrapped.
Merge remote-tracking branch 'upstream/master' into PartiallyFixedGeneralCovariance

Conflicts:
	statismo/PosteriorModelBuilder.h
	statismo/PosteriorModelBuilder.txx
	statismo_ITK/itkPosteriorModelBuilder.h
removed duplicated definition of PosteriorModelBuilder
Added documentation.
Merge branch 'PartiallyFixedGeneralCovariance' of https://github.com/tom-albrecht/statismo into tom-albrecht-PartiallyFixedGeneralCovariance