Add thermodynamic sea ice code

[ntlewis]

Adds thermodynamic sea ice code following Zhang et al. (2022).

Test case can be found here: exp/test_cases/thermodynamic_sea_ice/grey_thermo_ice_test_case.py

[ntlewis]

@sit23 @daw538 Would it be possible for one of you guys to take a look at this at some point? Thanks :)

[daw538]

Broadly looks fine to me. Most of the code is adding functionality rather than editing the existing model so happy to okay if it works as intended.

Probably don't need the lines that comments simply stating !ntl added as this would hopefully be self-evident from the diff in the PR.