Apparently GitHub changed their Markdown parsing at some point, had to change from one space to two spaces to create a nested bullet-point list again.

This is useful when one wants to load or save a single PCA model, for example when combining different shape and albedo models.
Also added python bindings for both functions.

Mostly we're using the _nnls function for blendshape fitting, but just for completeness, this function is now changed to use only Eigen as well.

Convenience function so they can easily be multiplied with a blendshapes matrix.

All matrix operations and solving the linear system is now done with Eigen matrices.
We use colPivHouseholderQr for now to solve the linear system, which doesn't have any requirements on the matrix.