Added ReducedVarianceModelBuilder, together with examples and wrappers.

The existing modelbuilders already had a feature for reducing the dimensionality. This feature is removed, as this functionality is now taken over by the ReducedVarianceModelBuilder.
Added unit tests for ReducedVarianceModelBuilder. Fixed bug in computation of variance
Merge branch 'reduced_variance_model_builder' of https://github.com/marcelluethi/statismo into test_marcel
updated the example so it saves the reduced dimensionality model