Issue a warning if there fieldmaps but they were not applied to any sequence

[yarikoptic]

It is unlikely to have a study where fieldmaps are collected just for the sake of collecting them. I think it would be valuable to inform user that some fieldmaps left unused after assignined IntendedFor.

WDYT @pvelasco and other @nipy/team-heudiconv ?

[tsalo]

I think a warning is reasonable, but FWIW I've seen a lot of datasets with unused field maps (e.g., multi-echo field maps where only the first echo's data are used, or acquiring both SE and GRE field maps).