morphablemodel::get_mean() for models with expressions

[patrikhuber]

For MorphableModels that contain a PCA expression model, we have to decide whether morphablemodel::get_mean() should return the combined identity + expression mean. If yes, that would mean that the result probably wouldn't be a neutral face.

[patrikhuber]

Changed this in b8be436 to return the overall, combined mean now. I think that's the more sound way.
If someone wants the obtain the neutral-only shape mean, they can use get_shape_model().get_mean() and sample_to_mesh.