--fisher-z in seed_corr

[shaharEd]

it seems like the fisher-z correction in ciftify_seed_corr happens only to volume values. in line 224 of ciftify_seed_corr:
if settings.fisher_z: run(['wb_command', "-volume-math 'atanh(x)'", nifti_Zcorr_output, '-var', 'x', nifti_corr_output])