Add code for subcommands

[ajtritt]

• Add place to register subcommands in top level init
• Update TOML to point to master command entry point
• Rename P(r) command functions

Motivation

Since we are adding more scripts/commands for re-use, I restructured the package for organizing these commands as subcommands under one main command.

How to test the behavior?

After installing the metfish package, the metfish command should be available. Calling it with no arguments will reveal the subcommands.

$ metfish
Usage: metfish <command> [options]
Available commands are:

 SAXS
    calc-pr         Calculate P(r) curves for PDB structures

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running ruff from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@48207de). Click here to learn what that means.

Additional details and impacted files

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[oruebel]

Looks good to me