dwipreproc: Revise output header key-values
dwipreproc: Revise output header key-values

- Retain some key-value pairs that were previously being erased, as they are still relevant for interpretation of the output data and may conceivably be utilised in subsequent processing.
- Retain phase encoding information if EPI distortion correction is not performed, since this may be relevant information for some subsequent processing to address such distortions.
- Write the DWI phase encoding table to key 'prior_pe_scheme', so that it is stored in the header for provenance but will not be identified and utilised by any subsequent commands.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3