Delete never-used mdarray and mdframe

[PhilMiller]

These aimed at being nice abstractions for various forms of multi-dimensional time-series data that we'd handle. However, they never ended up finding use in the rest of the codebase. So, they're just more code we're carrying and testing, without added value.

Removals

• mdarray and mdframe, along with their tests

Testing

1. CI

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️