Optionally update existing model with equations to AMR

[brandomr]

Per @YohannParis there is a need to use /equations_to_amr to re-run equations for an existing model. To support this, the endpoint should optionally take model_id. If provided, the resulting AMR would update the model in place instead of creating a new one.

[YohannParis]

It's not entirely accurate, but still right.
On the HMI we create an empty model (with an id, name, and description) before letting the user open it and entering equations, therefore we need to update this "empty" but already existing model.

[brandomr]

@YohannParis got it--so there won't be existing metadata that needs to be brought over. It's just updating the empty shell with something real

[YohannParis]

@YohannParis got it--so there won't be existing metadata that needs to be brought over. It's just updating the empty shell with something real

You are right! Just an id, name, description will be present.