distortion correction fails for VE data

[mcraig-ibme]

This is related to the difficulty in obtaining a PWI prior to vessel decoding. The PWI is required for EPI_REG.

However since the encoding cycles typically include a 'control all' and 'label all' image, it should be possible to implement this for VE data.

[mcraig-ibme]

One issue here is that we do not automatically know which of the VE volumes is the control and which the tag-all. We could assume the default (with a warning) first volume is control, second is tag-all.