Need for a schema update for default schema

[pavlis]

It is mainly my fault but new stuff I've been adding to MsPASS has expanded the Metadata namespace way beyond our current default schema defined in mspass.yaml. Key changes are:

1. Database adds some keywords that are not in the current schema. An example is "abortion" which is a boolean now always set True of False by the reader. (It might is "is_abortion")
2. The mspasspy.algorithms.snr module computes a long list of metrics that are not in default schema.
3. The new mutlichannel correlation (son of dbxcor) module I've been working on adds a whole bunch more standard keys.
4. The entire family of deconvolution operators use keys we don't have defined in the stock schema.

There are almost certainly others. e.g. I'm pretty sure the new ML code @JiaoMaWHU is working on adds some new stuff.

This actually raises two different issues I think the group needs to discuss:

1. General issues about how we should be handling the schema issue. With default use undefined key-value pairs are irrelevant. They only surface if you alter the defaults for read and write to the database.
2. Something we discussed a long time ago but never closed on is the need to write a parser that would convert the schema definitions to an rst format table that would display the content of the yaml file as a human readable table. If that existed there would be more motivation by any developer like me to update the schema file when adding a new algorithm.

We should perhaps have this as an agenda item on our next weekly meeting.