gh-782: Add benchmarks for `glass.fields`/`glass.harmonics`

[connoraird]

Description

Add benchmark tests for glass.fields and glass.harmonics for any functions which have both been identified as worth benchmarking in #790 and do not return Generators. Generator functions will be handled in #809.

Closes: #782, #813

Checks

• [x] Is your code passing linting?
• [x] Is your code passing tests?
• [x] Have you added additional tests (if required)?
• [ ] Have you modified/extended the documentation (if required)?
• [ ] Have you added a one-liner changelog entry above (if required)?