Incremental bug fixes for ciftify_clean_img and cifti_vis_PINT.
[FIX] heatmaps in cifti_vis_PINT have white lines in correct place (in newer python)
[FIX] ciftify_clean_img doc string now clearer
[FIX] ciftify_clean_img does not assume that nifti TR's are in ms
[FIX] ciftify_clean_img now exits gracefully when confound.tsv input is missing
[FIX] all meants functions now don't exclude voxels/vertices with means of zero