add base.context.dump to save experiment config as metadata

[vlap]

Adds toyaml (and to_yaml alias) Jinja filter to dump SE context as a yaml file (via yaml.dump).

[vlap]

hi @uwefladrich .

I am implementing "saveic" in ECE4. To make saved initial conditions fully self-documenting and traceable, I'd like saveic to also save a snapshot of the experiment config that defines the model setup (component list, resolutions/grids, forcing settings, and dates) that generated those initial conditions.

I did not find an easy way to dump from ScriptEngine in-memory context. With the new toyaml filter I can create
experiment-config.yml.j2 template

       {{ {'base.context': {'experiment': experiment, 'model_config': model_config}} | toyaml }}

and render it

            - base.template:
                src: "experiment-config.yml.j2"
                dst: "saveic/experiment-config.yml"

I hope this PR makes sense to you. Let me know if there is a better way to achieve the same.

[vlap]

when testing, I realized implementing a toyaml filter is complicated by ! tags, which are not rendered well.

So, I decided to add base.context.dump, a counterpart of base.context.load

[vlap]

I thought about copying the input file but it's fragile (cli args position uncertain, can be more than 1). So, base.context.dump is needed

   ```bash
   se my-user-settings.yml ../platforms/marenostrum5.yml my-experiment.yml scriptlib/main.yml

Also, some meaningful context, added via base.include can be missed (such as model tuning presets).

It can also filter out platform and job context to the model physics and experiment parameters.

base.context.dump is not meant to create a SE yml files with - base.context, just pure YAML, which can be loaded back into the context directly in downstream scripts using base.context.load.