Cleanup before package registration

[dfridovi]

Cleanup and polishing before package release and registration. Todo items:

• apart from module file, all other files should use file_name.jl instead of FileName.jl
• keep src/ to be the module file and only the minimal other files needed to run the algorithm, and no examples or tests
• tests go in test/runtests.jl
• put experiments or examples in a separate submodule. you can follow how I did it in MCPs.jl to handle the separate dependencies
• cut out unnecessary dependencies, and make sure tests and examples have isolated dependencies that do not pollute the main package dependencies
• update the CI following what I did in MCPs.jl
• we probably should rename the package to something less likely to have naming collisions and more likely to get approved by the Julia folks. Maybe SecondOrderZeroSum.jl