MNT: Only download fsl_course_data category by default

Default behaviour, when using the default manifest, is now to only present/download files in the fsl_course_data category. This behaviour is only active when using the default manifest, and can be overridden by specifying --category all (although at the moment, all files in the default manifest are in the fsl_course_data category anyway).

Also prefix downloaded files with their category to reduce the likelihood of collisions across different categories (e.g. registration.tar.gz from FSL course and from grad course). This does not prevent collisions in extracted paths though.

Behaviour of the --force option has also changed (primarily for testing purposes). It now causes all selected categories/modules to be downloaded (honouring the above rules).