Refactor IERS CSV parser

[AngusGMorrison]

Refactors the IERS CSV parser in preparation for delta UT1-TAI calculation.

In particular:

• Separates the domain model, EarthOrientationParams, from the input format.
• Moves parsing code to submodule iers, keeping the domain model in lib.
• Removes unused fields from Record, which I'll happily restore once they're needed, but I got sick of clippy yelling at me.
• Skips the intermediate Records implementation, moving directly from an iterator of Record to columnar EarthOrientationParams. This relieves some weirdness around when exactly to move between the Option<f64> read from the CSV and the f64 required by EarthOrientationParams.
• Does not disambiguate "Type" columns. These aren't used, and were previously distinguished by successive string replacement, which is extremely costly on 4 MB strings.
• Removes a number of unused EOP fields, which, again, I'm happy to add back in as and when we need them, but are currently noise.
• Marks mod iers as #![cfg(test)] to silence clippy until the code is used elsewhere than tests.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.46154% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 98.13%. Comparing base (bb12229) to head (a7dbb9b).

Files	Patch %	Lines
crates/lox-eop/src/lagrange/eop.rs	0.00%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #91 +/- ## ========================================== - Coverage 98.13% 98.13% -0.01% ========================================== Files 55 56 +1 Lines 19560 19427 -133 ========================================== - Hits 19195 19064 -131 + Misses 365 363 -2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.