Refactor of openfmri code

- code relevant to openfmri is now in the openfmri folder
- the idea is to have one script per dataset, with all the required information (bad data, missing data etc...) and to be able to download, preprocess, and process from a single script
- ds001 & ds002 are available
- external/nisl was replaced by a full up-to-date version of nisl
- pypreprocess specific code from nisl should have been moved to datasets_extras in the root_folder