dwipreproc: More topup / eddy alignment work
dwipreproc: More topup / eddy alignment work

- Check to see if both hears contain an echo time key-value, and give a warning if they differ (e.g. if one were to use a shorter TE for acquiring spin-echo EPI images just for estimating the inhomogeneity field, the technique employed here would no longer work, since topup would be estimating squared errors between those volumes and a b=0 volume from the DWIs).
- If the DWIs and spin-echo EPI images are not defined on the same voxel grid, issue a warning, and re-sample the spin-echo EPI images onto the grid of the DWIs; this is necessary to enable concatenation of the first b=0 DWI volume at the start of the topup input.

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