This PR introduces a new plugin infrastructure for the OPS CLI. This is a major API break, and all externally-developed plugins will need to update for it.

Fortunately, the update procedure is simple: Previously, plugins had to be one per module (file), and the modules needed to define the variables CLI (to label the command), SECTION (the help section to use), and optionally REQUIRES_OPS (the minimum OPS version).

The new plugins require importing paths_cli.OPSCommandPlugin, and then creating an OPSCommandPlugin instance, which takes the required variables command and section, and the optional variables requires_ops and requires_cli (which defines the minimum CLI version.) Note that this OPSCommandPlugin instance must be assigned to a name in the module (it needs to be listed in the module's vars.) I happen to choose the name PLUGIN in the examples, but the specific name doesn't matter.

For examples on how to do the update, see changes to files in the commands directory in this PR.

The major advantages of the new plugin system are:

1. It is possible to define more than one plugin in a given file.
2. The new approach is not narrowly focused on subcommands of the CLI. The plugin infrastructure could be used for plugins for other tools, such as in Simulation Setup Wizard #41 or in compile command: Simulation setup with YAML or JSON #43.

For developers who want to support for OPS CLI 0.3 (where this will be included) and earlier versions, the two methods can be used simultaneously, but both methods must be included in order for plugins to work with all versions of the CLI (i.e., unmodified old plugins will not load with new versions of the CLI).

Tasks:

• Code
• Tests
• Migrate commands
• Migrate miscellaneous (examples for writing plugins; plugins used in testing)
• Update docstrings on plugin management tools
• Update docs on writing plugins