add require_variance to model_interface look_at signature

[benlonnqvist]

This PR resolves issue #651.

In addition, this PR adds traversal paths for require_variance for non-neural task models (Behavior, ProbabilitiesMapping, Temporal).

[deirdre-k]

Hi @benlonnqvist, is this PR still in progress? If not, should it be closed? Thanks for the contributions!

[benlonnqvist]

Hi @deirdre-k, thanks for the message. This PR is technically still in progress and while PR #917 fixed the issue about being able to call microsaccades, the signature issue in model_interface / elsewhere still seems to be open.

We could either:

1. wait for me to get to this at some point in the future (I see this as quite a low priority item for me personally)
2. close this and leave the issue require_variance missing from the signature of look_at #651 open with a note pointing to this PR as a somewhat-approved way of resolving the issue

Thanks!