"standardization" of task metadata across modality

[Remi-Gau]

Was having a look at the ephys BEP and realized that even though this is a modality that uses the task entity the BEP does not menton some of the typical task related fields that other modalities have.

But I then realized we do have this problem in the current spec already ( see below).

So I would suggest to make more consistent what task-related fields are available for all modalities.

The fields in question are the following (I may have missed some):

      "Instructions": "RECOMMENDED",
      "TaskDescription": "RECOMMENDED",
      "CogAtlasID": "RECOMMENDED",
      "CogPOID": "RECOMMENDED",

And they not mentioned for :

• task based PET
• behavioral

I suggest opening a PR to add those.

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This also highlight the need to do some refactoring of some of the task aspects that are common across modalities.

Something that is related to what @robertoostenveld mentioned in #575

After adding those I suggest, we should probably find a way to do this refactoring.