Improve handling of the metadata

[Remi-Gau]

A few basic functions that I would find useful, from the perspective of creating pipelines that generate/process derivatives in a BIDS-compliant manner:

• A function for changing a BIDS filename, based on changes to specific key/value pairs (i.e., changing, adding, and removing key/value pairs to a name, and maintaining the proper order of fields in the name).
• A function for changing one or several fields in a .json metadata file (e.g., to generate a new metadata file after applying some transformation / preproc step to a dataset)
• A non-private version of bids-matlab's get_metadata function. I frequently find myself wanting a simple function to check a specific metadata field of a single file, without having to run both bids.layout and bids.query. I understand the rationale for keeping get_metadata private, but from my perspective it would be very useful to have a public version of something like this command.

Originally posted by @bmdeen in #60 (comment)

See also comments:

#60 (comment)

[Remi-Gau]

Originally posted by @gllmflndn

@bmdeen: good point about get_metadata: we should have a user-level helper function to query the metadata of a given file.

[Remi-Gau]

Add a function for changing one or several fields in a .json metadata file (e.g., to generate a new metadata file after applying some transformation / preproc step to a dataset)