Enable import for HCP pre-processed DWI

Should create a flag to enable import from HCP pre-processed DWI.

This flag should force-skip dwi pre-processing steps, and instead link or copy over data into the work folder, and set `prevstep=fromhcp_regT1_gradCorr`. Then, the processCleanupBids function will set-up all the bid-style links as per a typical prepdwi run.

From the HCP T1w/Diffusion folder, can map to:
```
data.nii.gz -> work/sub-001/dwi/fromhcp_regT1_gradCorr/dwi.nii.gz -> sub-001_dwi-space-T1wGC_preproc.nii.gz
grad_dev.nii.gz -> work/sub-001/dwi/fromhcp_regT1_gradCorr/grad_dev.nii.gz -> sub-001_dwi-space-T1wGC_preproc.grad_dev.nii.gz
bvecs -> work/sub-001/dwi/fromhcp_regT1_gradCorr/dwi.bvec -> sub-001_dwi-space-T1wGC_preproc.bvec
bvals -> work/sub-001/dwi/fromhcp_regT1_gradCorr/dwi.bval -> sub-001_dwi-space-T1wGC_preproc.bval
nodif_brain_mask.nii.gz -> work/sub-001/dwi/fromhcp_regT1_gradCorr/brainmask.nii.gz -> sub-001_dwi-space-T1wGC_brainmask.nii.gz
```
and bedpost:
```T1w/Diffusion.bedpostX -> work/sub-001/dwi/fromhcp_regT1_gradCorr/bedpost.bedpostX ```