"set_metadata" parameter

[hyanwong]

Linked to #203 and #237 (comment). At the moment, tsdate, as well as changing node times, also changes node metadata to add (unconstrained) means and variances. It would be useful to have the ability to (a) not add to metadata and (b) specify whether or not node metadata should be stomped on, if it exists.

Since this is the only metdata that tsdate touches, I suggest that as a first step, we could have a parameter set_metadata, save_metadata, store_metadata or even force_metadata with the following values:

• False: do not touch any metadata columns in any tables
• True: always add metadata to the node column, changing the schema and overwriting any existing node metadata
• None (default): try to append the existing node metadata, overwriting the tsdate-specific fields if they exist. A minor wrinkle is if there is binary node metadata (i.e. without a schema) and the metadata column is not empty (see Set node metadata schema #203 (comment)). In this case we won't know how to adjust the node metadata properly, so we should either raise an error or emit a warning. I suggest we should raise a warning, but it would be a pain to have run tsdate all the way through only to raise an error at the end, so we could check before running the main tsdate routine, and raise the error early?

[hyanwong]

For backwards compatibility #303 is slightly more sophisticated and if the null schema is set, tries to interpret it as JSON (and if it succeeds, sets the schema to a permissive JSON type).