add another postprocessing step to HBN study data -- rename AP/PA fmaps

[akeshavan]

the HBN fieldmap files aren't consistently in BIDS format -- for the files where we have to read the .json file to figure out the fieldmap direction, we need to save the file with _dir-AP and _dir-PA in the filename so that pybids can find the right files in the io module. Most of the Site-RU subjects don't have the right naming scheme (e.g. sub-NDARUM797TFA from RU)