Implement `B0FieldIdentifier` in `FieldmapEstimation` object

[oesteban]

To facilitate the implementation of bids-standard/bids-specification#622 if that comes through, with the following:

1. Extend FieldmapEstimation with an id property, and to keep a register of unique IDs (to ensure there're no duplicities).
2. Read the id from the BIDS metadata. If the id is not present, generate one automatically and forward it to the derivatives.
3. Make sure the id makes it to the subworkflow name to ensure correct Nipype performance and avoid collisions.