Add default template and ability to derive templates

[cmungall]

schemasheets is powerful and flexible with its template mechanism

It would be useful to have some standard templates:

• for people to start filling in de-novo schemas
• for use as a starting point for linkml2sheets

These could be standard TSVs that are distributed along with the. pypi package, with convenient commands for seeding files with these.

When going from an existing linkml schema it might be useful to also autogenerate a template that includes all used metaslots

See also

• #41