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.

When switching the shape and blendshape fitting to use the new Eigen models, these matrices weren't correctly mapped to cv::Mat. Eigen uses col-major storage order by default, and OpenCV row-major - so we now convert to a row-major Eigen matrix before mapping to a cv::Mat.
This caused the fitting to produce faulty/meaningless results since the switch to Eigen.
The output of the linear shape and blendshape fitting functions are now again identical before and after the switch to Eigen. (after 50 iterations, the result is marginally different, most likely because of small rounding differences.)

* ImageCost throws on instantiation if the given model doesn't contain a colour model
* Changed the default parameter of fit-model-ceres to sfm_shape_3448, which will need to be obtained
* Additionally the app checks explicitly for has_color_model() and will exit if it hasn't got one.

Otherwise an Eigen expression (CWise...something) will be returned and subsequent code will fail.

This fixes a crash - it should obviously return a matrix, not a vector.

Install if the Eigen from the submodule (3rdparty directory) is used. This is the case, except when a user specifies their own EIGEN3_INCLUDE_DIR - in that case, we don't want to install the headers.