Introduce CliArgs (new design) and use to further generalize ThreeBusBasicJson example

[PhilipFackler]

Description

This is another step toward a generalized application to replace examples.

Proposed changes

Use a new class CliArgs to easily define command line options.

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

Further comments

Note that this is a draft PR. I wanted to get feedback on the design of CliArgs, and (if we want to keep it) I need to add documentation and testing for it. I would also add the capability to handle positional arguments.

[pelesh]

This is very welcome and long overdue update. Thanks for taking initiative on this. A few comments:

• Please document new functions with more detailed comments in the code.
• Consider separate source/header files for different classes. Some of the classes are only a few rows but it might be a good idea to stick to "one class per file" rule.
• I made some style suggestions that clang-format will not fix.

[pelesh]

I suggest we put classes ArgValue and ArgVector in separate header and source files.