Add standard defined key for points to echo centre / begin time etc.

[wtclarke]

@ctr suggested in bids-standard/bids-specification#1846 that there should be a standard/bids defined field for the number of points to discard from the start of the signal to reach the echo centre.

• What should the naming convention be for this? Is there already a convention in DICOM?
• Should it encode number of points (int or float?) or a time? The former is easier to use and implement, but the latter could then also be used to define begin times (i.e. time after the FID start or echo centre that the acquisition starts).

Personally I would go with encoding time and would suggest EchoCentreOffset or 'FIDTimeOffset`.