Allow modification and deletion of nested associations

[GianlucaFicarelli]

Description

Following #545, allow to

1. modify (replace) the associations with a PATCH request when they exist already
2. remove the associations without deleting the main entity (same PATCH request as in the previous point, but with empty list of associations)

Additional context

Currently the nested associations can be created only when missing.

Examples of nested associations, see the defined list:

• EModel.ion_channel_models
• TaskConfig.inputs
• IonChannelModelingCampaign.input_recordings
• SkeletonizationCampaign.input_meshes
• Activity.used_ids
• Activity.generated_ids

Acceptance criteria

• It's possible to replace the associations with a patch request even when they exist already
• It's possible to remove the associations