Add hypothesis strategy for SGRID dataset generation

[VeckoTheGecko]

Description

This PR:

• Adds a hypothesis strategy for SGRID dataset generation under parcels/_datasets/structured/strategies.py
• Moves the strategies such that they are part of the parcels module (instead of the tests module) - tests.strategies -> parcels._strategies
  • Since the _datasets folder is in the Parcels package, I think this structure makes the most sense
  • Informatively errors out if someone tries to import strategies without having hypothesis installed

This in future will allow us to have more property based testing extending to full simulations - which would be really helpful for both improving the robustness and maintainability of the Parcels test suite.

Checklist

• Closes None
• Tests added
• This PR targets the correct branch (main for normal development, v3-support for v3 support)

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
  • Describe how you used it (e.g., by pasting your prompt): For writing tests/datasets/test_strategies.py

[erikvansebille]

Looks good; just two questions below

[erikvansebille]

Why is this needed? Wouldn't hypothesis simply be part of our Pixi install?

[VeckoTheGecko]

Its part of our Pixi install, but its not a Parcels run dependency (i.e., its not in pixi.toml::run-dependencies or the recipe.yaml/pyproject.toml.

This is our first "optional dependency" for Parcels (i.e., a part of the codebase that needs a specific package in order to fulfill a function, but where it doesn't make sense to include it for everyone since most people wont use the specific function).

People doing conda install parcels then import parcels._strategies will encounter this more informative error message.

[erikvansebille]

OK, I see. But then (in a next PR?) fix the type-o preffered to preferred?

[erikvansebille]

Perhaps this is just good practice, but I'm surprised that the strategies are moved out of the tests directory. Would they not more logically belong there?

[VeckoTheGecko]

Yeah - good question.

Most of our dataset generation code we have shipping with Parcels via parcels._datasets. This means that users can have easy access to Parcels-compatible example datasets and dataset generation for posting issues etc. . When adding strategies for creating example datasets, I needed to use existing strategies that were in the tests module. Since I cant put code in parcels that depends on the tests module - the options I saw were:

1. Leave the strategies where they are in tests and carve out a part in tests/... for housing these dataset strategies. This would mean that some datasets are used via parcels/_datasets/... and some via tests/... which I felt was confusing for devs
2. Move the dataset generation to tests - foregoing the benefit of users having easy access to datasets
3. Move the strategies to parcels

I thought (3) was the best. It might seem weird to be putting "test" code in the parcels release, but its actually not that weird - some projects even put their whole test suites in the release itself.