update DB with correct image info

[rfdougherty]

If a user stops a scan prematurely, the actual number of timepoints will differ from the prescribed number. We should update Epoch.num_timepoints and Epoch.duration to reflect the actual values. (Epoch.prescribed_duration will remain unchanged.)

For mux (multiband) scans, the number of slices is currently computed correctly from the header, but we should allow for the fact that it might need to be updated after the recon is done. So we should update Epoch.num_slices as well.

Both of these can be updated after the NIFTI is generated. Modify dicomutil.DicomAcquisition.convert and pfile.PFile.to_nii to return the actual num_timepoints and num_slices, and the relevant processor pipelines should be modified to push this info to the Epoch.

[kevlarkevin]

processor checks size, mm_per_vox, num_slices, num_timepoints, and duration after parse, and updates the Epoch info in the DB.