Handling of multi-echo data

[tsalo]

I fully expect that the majority of datasets that get run through this package will be multi-echo. I think we'll need to figure out how to handle those data for a number of steps, including:

• RETROICOR: Run independently on each echo?
• HighCor: Use a combined phase image? Maybe the weighted average from warpkit.
• Phase regression: ???
• LayNII: Write out each of the echo-wise jolt and jump files, but also maybe an unweighted average file as well.
• GIFT: ???