Provide easy way to loop over files as structure

[nbeliy]

In actual state user have an access either to the flat list of files using query, or to full unfiltered layout. It can be helpfull to provide user a way to loop all datafiles returned by query with access to corresponding structure.

It can be done using already existing indexing:

finfo = bids.internal.return_file_info(BIDS, data_list{iFile});
file = BIDS.subject(info.sub_idx).(info.modality)(info.file_idx);

However return_file_info rely on find, and so it is time consuming.

The easiest solution (that I see), is to create a new function query_struct(BIDS, query) which will replicate existing query(BIDS, 'data', query) but returnsa list of found file structures. To optimize, the query("data") will call query_struct(BIDS, query) and retrieve the filenames.

Alternatively, an iterator can be implemented:

iterator = bids.BEGIN(BIDS, query);
do
  file = iterator.get();
while iterator.next()

BEGIN creates in memory a list of file infos, get returns corresponding file structure next moves current file info to the next in list