Define and use contents of `<SchemaInfo>`

[StokesMIDE]

Some schemata have a header element, <SchemaInfo>. The intention was to have it contain metadata, but was never fully developed; it's arguably useful to humans, but is currently ignored by the schema parser.

Two things should be done:

1. Specify all the child elements. Currently, schemata use <Author> and <Description>. Maybe we should use the standard set of Python module 'dunder' variables as the model.
2. Get Schema to parse the info. Some can be ignored and left for humans, but a few things are useful.

This arguably ties into #110.