added new Example, which illustrates the use of the PartiallyFixedModelBuider to enforce that a fitting result matches the given landmark points.
changed version number to 0.81 (back from 0.9) to mark an intermediate release
Introduced Posterior Model Builder, which replaces the PartiallyFixedModelBuilder.

We have refactored the PartiallyFixedModelBuilder such that the code closely corresponds to the theory and notation used in our Media paper "Posterior Shape Models". To make the connection more clear, we have saved it under the new name PosteriorModelBuilder. The PartiallyFixedModelBuilder is deprecated and will in future versions be removed.
Moved initialization of constants out of class definition.

Defining constants of type double directly in the class declaration seems to be disallowed in Visual Studio.
Merge pull request #17 from marcelluethi/landmark-constrained-fitting-example

Added new Example for fitting a landmark constrained model
Merge pull request #18 from marcelluethi/release-0.81

Preparations for an intermediate release 0.81
added a more general application for building vtkPolyData shape models
Merge branch 'marcelluethi-posterior_model_builder' into main_master
Merge pull request #19 from marcelluethi/posterior_model_builder

Introduced Posterior Model Builder, which replaces the PartiallyFixedModelBuilder
Merge branch 'master' of https://github.com/statismo/statismo into main_master
added method PointToVector to representer interface
Added GaussianProcess registration functionality.
change refLandmarks to modelLandmark in assert
Made use of std::async optionally

The feature, which is not supported by all compilers, needs to be
explicitly enabled with CMake.
Merge pull request #22 from marcelluethi/gp_registration_statismo_081

Gaussian Process registration integrated into statismo