OO-ify the code generation code

[mccalluc]

• fix More OO for templates #162 and clean up a lot of duplicated code
• fix In template, error if code block substitution fails #156: Stonger tests of template filling
• Rename from template to code_generators: the templates are just a means to an end.
• ... and put the template machinery in its own file.
• Named tuples instead of dicts for AnalysisPlan and AnalysisPlanColumn: Complements Type checking with pyright #164

For the reviewer:

• The diff is going to be hard to read, but if you just look at the result, does the code organization make sense?
• Are there other obvious failure modes that should be tested on the templates?

[pdurbin]

I didn't run the code but the new version looks fine to me.