Split requirements.txt to separate files for clients/development/docs/agents

[BrianJKoopman]

Description

This splits out the requirements file so users can install a minimal amount of packages just required for running ocs clients, while optionally being able to install all packages needed for development, etc.

I've updated the documentation to describe the separate requirements files. We default to clients and docs dependencies. Individual agents are now managing their own dependencies, so those will be installed only in the specific Agent containers. Lastly there's the dev.txt file, which includes pytest. We use that in the Dockerfile for building ocs images since we run the tests in the CI pipeline. This could be split out if desired, but I'm not sure it's worth doing.

Other notes:

• Removed ipywidgets requirement by also removing old ocs_widgets.py module.
• Removed msgpack, as it didn't seem to be used in anything -- please correct me if I'm wrong (it does seem to get pulled in by some other packages, but maybe we don't need to explicitly install it)

Motivation and Context

Closes #92

How Has This Been Tested?

I'm working on testing this. Tests pass so far though.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• Unless I am preparing a release, I have opened this PR onto the develop branch.

[BrianJKoopman]

Many of the changes in the works here have happened gradually over several PRs.

• A readthedocs config file was introduced in Create rtd config file, split requirements, and pin versions #235
• Documentation build requirements were split to a separate requirements file, also in Create rtd config file, split requirements, and pin versions #235
• crossbar was removed from the requirements file in Remove crossbar from main requirements.txt #173
• The need to optionally pip install crossbar was pointed out in the documentation in Deal gracefully with crossbar not being found #208
• Modules required for testing were split out in Build and Deploy to PyPI #243
• Build and Deploy to PyPI #243 also publishes ocs to PyPI, which will automatically pull only the dependencies needed to run ocs

Given that progress, I don't think we'll be reducing the number of packages much further from here. So I will consider #92 resolved and also close this draft PR.