Add bids-walker.js support for Brainlife neuro/anat/t2starw datatype

[astewartau]

This supports converting BIDS MEGRE and T2starw datasets into the Brainlife neuro/anat/t2starw datatype in support of techniques such as QSM.

It assigns part datatype tags such as part-phase, part-mag, part-real, and part-imag, with other tags such as echo stored as regular object tags.

I also included some other minor cleanup including changes that prevent erroneous error messages from appearing about unrecognised data files.