add facility to generate entity relationship diagram for tap schema

[pahjbo]

This would be useful for the documentation

Implement via plantuml or mermaid?

[pahjbo]

will be released as 0.5.29

still need to

• get multiplicities right
• mark primary keys (and if natural key)

probably needs ivoa/TAPSchemaDM#3 ivoa/TAPSchemaDM#4 to do well