NF: generate sforms in NIFTI dataset
ENH: detect NaNs in feature selector
RF: better information message when external not found
NF: support AFNI binary as external
RF+OPT: faster string comparisons and checking whether a cell contains just strings
RF+BK: return stat labels in a more standard (NIFTI-like) format
RF: change unit test to support new stat code format
SML: minor changes in unit test
RF: use new spherical_neighborhood in searchlight test