Make --model parameter of host application more flexible

[hannobraun]

Currently, if you run fj-host -m my-model, it tries to find a model in models/my-model. This is convenient for use in this repository, but it is less convenient for other use cases.

Here's how I could see that being fixed:

1. If no -m argument is passed and there is a Cargo package in the current directory, load that.
2. If no -m arugment is passed and there is no Cargo package in the current directory, fall back to current behavior.
3. If the parameter passed to -m is an existing path, attempt to load a model from there.

Labeling https://github.com/hannobraun/Fornjot/labels/good%20first%20issue, as this is a rather small change that doesn't require detailed knowledge of Fornjot. Some notes on the implementation:

• Arguments are specified in args.rs.
• Models are loaded in model.rs.

I think it would make sense to add the new logic in model.rs.

[hannobraun]

edit: Incorporated this comment into the issue description above.

---

In case anyone wants to look into this:

• The argument is specified in args.rs.
• The argument is then used in model.rs.

I think it would make sense to add the logic I described here in model.rs.

[hannobraun]

The issue description specifies "falling back to the current behavior", if all else fails. #123 is relevant here, as it would make said current behavior more sane.

[hannobraun]

Updated the issue description to make it more clear what can be done to address this issue.

[hannobraun]

I'm working on this.

[hannobraun]

I've looked into this, and it has turned out to be surprisingly difficult. The problem is, a model looks like any other Cargo package, so determining whether there is a model in the current directory isn't straight-forward. This could be solved by adding a special file to model directories, or special metadata to the Cargo.toml of models, but that's more complication for the user. I don't think that's worth it right now.

How models look is an area that is going to change a lot in the future, with the planned move to WASM (#71) and support for more modeling languages. Once that has settled down somewhat, I think it's worth revisiting this issue. For now, I've added it to the feature wishlist.