ENH: add measure sparsity error code.
Merge branch 'RPCA_BRATS' of github.com:XiaoxiaoLiu/TubeTK into RPCA_BRATS

Conflicts:
	Base/Python/pyrpca/examples/Low_Rank_Atlas_Iter_BRATS.py
	Base/Python/pyrpca/examples/TissueStatisticsValidation.py