CLI / BidsPipeline Argument for Fieldmap Correction

[khoffschlag]

Add an argument to the command-line interface (CLI) and BidsPipeline that allows users to specify whether fieldmap correction is required.
If the argument is not supplied, the default behavior will be to skip fieldmap correction.

If fieldmap correction is desired, the user must provide two acquisition identifiers to this CLI / BidsPipeline argument, such as:

• "LR" (i.e., left-to-right phase encoding)
• "RL" (i.e., right-to-left phase encoding)
  The acquisition identifiers depend on the filename of the MRI images. Providing these identifiers allow the BIDSGrabber to identify the files requested for fieldmap correction.