Feature request: function to add new fields to dataset_description.json

[nbeliy]

When creating derivatives, BIDS requires to add GeneratedBy field. On itself it is not difficult, but it would nice to have a simple centralised script to generate such fields.

Something like:

update_description(BIDS, fieldname, data, append)

with append a bool that decides if field will be appended to existing field, or will replace it.

[Remi-Gau]

Very mcuh agreed.

This is part of creating / enhancing the "writing" aspects of bids-matlab.

This issue is subset of some the feature suggestions from @bmdeen in #79.

Will also have start having a look into this, as it could help create some utilities that could be useful for other things later.