Details about phase1/2 fieldmaps - order of operations (subtraction/PRELUDE)

[oesteban]

Follow the conversation that started around here #30 (comment).

[kimsin98]

As I explained in #30 (comment) , subtracting before unwrapping should result in the same results most of the time and fix cases that produce the out-of-phase "blobs" currently.

I opened a PR with the switch. #70