Option(s) or heuristic component to augment metadata in sidecars

[yarikoptic]

Initial use cases brought up by @dnkennedy.

Initial demand comes from the fact that Philips does not provide all desired metadata for fieldmap correction, so dcm2niix guestimates it and places into complimentary fields: EstimatedTotalReadoutTime and EstimatedEffectiveEchoSpacing. So it is up to the heudiconv operator to decide either he/she trusts those and wants to copy them (as is) to TotalReadoutTime and EffectiveEchoSpacing.

Another case -- to provide PhaseEncodingDirection based on PhaseEncodingAxis which dcm2niix extracts and some "heuristic", e.g. something in sequence name, like _dir-AP.

I believe we also had other cases where people want to be able to add metadata into side-car files... we need to collect them and see how to provide support. Most likely through some dedicated heuristic function.