DOC: fixed .nii.gz extension
RF: update of unit test for fmri dataset
NF: stack samples
NF: split half correlation measure
NF: cosmo_map2array
ENH: allow empty sample attributes
ENH: use map2array for general view of slices irrespective of data type
ENH: more stuff in .gitignore
NF+RF: general dataset stack in either sample or feature dimension
ENH+DOC: improved split half correlations