ENH: avoid resampling

if not necessary
Change fetch_adhd to handle the new format
Replace Haxby simple fetching by new functions
BUG: data_dir was not taken into account
Remove ADHD dataset fetching test as it basically test nothing.
Migrate Miyawaki fetching toward new API
Factorize loading code of ADHD
Move craddock atlas fetching to new system
Move fetch_nyu_rest to new API
Add regressor information for ADHD
Move Haxby dataset fetching function and remove obsolete code
Add labels for Harvard Oxford atlas
Add phenotypic information for adhd
Add stimuli fetching to haxby
Remove obsolete test
Remove resampling of ADHD dataset
Remove old test
Add back test functions for datasets
Fix _fetch_files docstring
Fix import. Fix docstring.