Add a way to add a variable to a dictionary using a complete specification

[tramora]

Fixes #401

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TODO Before Asking for a Review

• Rebase your branch to the latest version of dev (or main for release PRs)
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  • If there are any problems it is faster to iterate by building locally the API Docs

[popescu-v]

See the comments.

[popescu-v]

Some pending comments.

[popescu-v]

A few comments.

[popescu-v]

LGTM. Can be merged after rebase on dev.