Make a common templates directory for users to peruse

[beykyle]

This should include any useful templates we make, including the ones used in the test suite. It should be referenced in the contributing guidelines for users who would like to share templates that they have used.

[jared321]

1. Should users be able to query each package to see what templates it offers?
2. If so, should users also be able to get the set of keys needed to define the template and that define the basis of the problem's parameter space? Could that output include a one-line string for each parameter that is the official documentation for the key?
3. bfrescox.print_template_specification("elastic") or could help(bfrescox.generate_elastic_template) be complete?
4. Should we provide them direct access to the base template in the package or is it sufficient for them to just use the interface to create the template and inspect that result?

Potential answers to some questions investigated in prototype
https://github.com/bandframework/BFrescox-Internal/blob/main/DesignDocs/PotentialTemplateDesign.ipynb