feat: Add algorithm model validators

[jdeschamps]

Description

Following #354: final_activation was allowed to be set although it would break CARE, N2N and N2V.

This PR adds a Pydantic validator preventing final activation for these algorithms. It also takes the opportunity to improve the Pydantic models by moving the validators to a different module and using annotations for cleaner code.

• What: CARE, N2N and N2V do not allow final activation in the UNet.
• Why: This would otherwise break the algorithm training.
• How: Add model validators in the respective Pydantic models.

Changes Made

• Added: model_validators.py.
• Modified: Algorithms model.

Related Issues

• Fixes Validate UNet final_activation in N2V, CARE and N2N algorithm configurations #354

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• Code builds and passes tests locally, including doctests
• New tests have been added (for bug fixes/features)
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