Implement an automatic way to compute the number of subspaces
Gong, Boqing, et al. "Geodesic flow kernel for unsupervised domain adaptation." Computer Vision and Pattern Recognition (CVPR), 2012 IEEE Conference on. IEEE, 2012. Section 3.4 describes a way to compute the optimum value for the number of subspaces (called Subspace disagreement measure).
Would be nice to have that.