Refactor parameter caching + add Zygote tests

[SouthEndMusic]

Fixes #298
Fixes #301

#289 can be a follow-up. Gradients w.r.t. u for QuadraticSpline are not broken anymore, but they are slow as mentioned in the issue.

The refactor includes:

• Caching parameters is opt-in, so users need to make the active decision to cache parameters. It is documented that this is not compatible with modifying u and t. We could if desired allow this with a dedicated interface which updates the parameters automatically. When parameters are not cached, they are computed on demand with the same functions.
• The ReadOnlyArray wrappers are removed, which simplifies the type parameters. I also noticed an inconsistency where I forgot to do it for integral inverse interpolations in the first place
• Also safetycopy as an option is removed again

Zygote:

• I added Zygote tests for differentiation w.r.t. the interpolation input and w.r.t. the u of the data points for where that is supported (again).
• I've added some examples in DataInterpolationsChainRulesCoreExt.jl of how to speed up gradients w.r.t. u with analytical expressions. Speeding up more interpolation types this way could be a follow up

[SouthEndMusic]

How can I fix the Zygote compat?

[ChrisRackauckas]

You can only compat weakdeps or deps. So added as a weakdep if we want the compat.

[SouthEndMusic]

I've added tests for I think everything that was compatible with Zygote before. I've added some examples in DataInterpolationsChainRulesCoreExt.jl of how to speed up gradients w.r.t. u with analytical expressions. Speeding up more interpolation types this way could be a follow up