New EPI dewarping using a workflow

This new feature implements the EPI dewarping methodology from epidewarp.fsl,
as a workflow and including some enhancements allowing:
  * Use for dMRI
  * Optional registration
  * New EPI parameters support (parallel imaging, reverse encoding)

Additionally, some improvements on the FSL FUGUE's API have been included:
  * Added --saveshift argument support
  * Added --warp argument support
  * Improved the output listing methods for coherence
New EPI dewarp workflow (#525)

  * Removed useless comment
  * Changelog updated
Merge pull request #525 from oesteban/master

New EPI dewarping workflow, in a similar manner to eddy_current correction