Improve default argument handling

[nwatson22]

Closes #885.

Overhauls the way command line argument parsing and parameter passing is done.
When building a pyk-based command-line tool, for each subcommand of the tool, extend class Command . This subclass should contain all information about what arguments are accepted by that command (through providing its own arguments or inheriting from other Options classes), the default values of those arguments, the name of the command, and the help string for the command, as static fields. The values of those options for a specific invocation of that command are stored as non-static fields of a Command. The Command subclass contains the code that runs when that command is called in exec(). In addition, default values of arguments inherited from other Options classes can be overridden in the subclass.

The CLI class manages the tool's CLI options. It is constructed by passing in the name of every command subclass to be included in the tool. It can then build the whole ArgumentParser for the tool and process the arguments to instantiate a new *Command of the correct type and with the correct arguments.

Advantages:

• All information about a subcommand is consolidated into one place
• Default values specified in only one place
• Commands only have to be listed once, when instantiating CLI
• Setting up an argument parser with all subcommands is done automatically.
• Routing of requested command to its associated execution function is done automatically.

[ehildenb]

Going to merge this, and get it pushed through to all downstream repos, then can work on next batch of refactorings. Mostly because I have other CLI changes that I want to go through, and want ot try out the new interface for making those changes.

After we get this pushed through, we can discuss the next refactoring/design that we'd like to pursue here: runtimeverification/k#4188