Added ICA/PCA/Avg/Regressors preproc for fMRI.

Also:
- Added mrimage_utils overlapRatio, resampleNN
- Fixed accessor for rgba_t/rgb_t
- Modified PCA to for dimension reduction options, either use variance
  or set a hard number
Merge branch 'master' of github.com:MicahChambers/npl

Conflicts:
	lib/statistics.cpp