Develop testing infrastructure to allow project-agnostic testing of flowsheet interfaces

[lbianchi-lbl]

Currently, my plan is to add a pytest plugin to this package (so that it's installed and available alongside the library code) that runs a series of standard validation tests for all registered interfaces.

The plugin would be opt-in, i.e. would only run when the identifier for a specific source of interfaces is given. In the first iteration, this would be the entry points group. As an example, for WaterTAP, the entry points group is watertap.flowsheet, so the plugin would be invoked as

pytest --flowsheet-entry-points watertap.flowsheet