Add FullDelayESN: model with simultaneous input and state delays

[MarcoCerino23]

I've added the FullDelayESN model.

This model combines both delay mechanisms independently, allowing for distinct configurations:

• Input Delays: Controlled by num_input_delays and input_stride (operates before the reservoir).
• State Delays: Controlled by num_state_delays and state_stride (operates after the reservoir).

I've implemented the struct and added comprehensive tests (all passing locally)

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the [contributor guidelines](https://github.com/SciML/.github/blob/master/CONTRIBUTING.md), in particular the [SciML Style Guide](https://github.com/SciML/SciMLStyle) and [COLPRAC](https://github.com/SciML/COLPRAC).
• Any new documentation only uses public API.

[MartinuzziFrancesco]

This is essentially ready to go! I've just added a couple of minor comments. Also, I think that naming wise we could change your FullDelayESN to be named as DelayESN, and change the current DelayESN to be called StateDelayESN. Does it makes sense?

[MarcoCerino23]

Absolutely, I totally agree! That naming convention makes the hierarchy much clearer and more intuitive.

Here is a summary of the changes in the latest commit:

1. Renaming:
   • FullDelayESN → DelayESN (since it covers both delay types).
   • The previous DelayESN → StateDelayESN.
2. File Consolidation: As suggested, I moved all three delay models (InputDelayESN, StateDelayESN, DelayESN) into a single file: src/models/esn_delay.jl. I did the same for the tests in test/models/test_esn_delay.jl and removed the obsolete files.
3. Refactoring: I updated the new DelayESN struct so that the state delay layer is handled inside states_modifiers.

I believe everything is clean and ready now!

[MartinuzziFrancesco]

awesome, we merge as soon as tests pass!