Explicit construction of fixedPointSet with 0 PointData from reference mesh.

Before this change, it was assumed that there is a 0 scalar field defined on the reference mesh. With this change we remove this requirement and explicitly construct a new fixedPointSet from the reference mesh. We add the 0 scalar field explicitly. This example should now work with any shape model.
Merge branch 'fix_lmconstraint_model_builder' of https://github.com/marcelluethi/statismo into marcelluethi-fix_lmconstraint_model_builder