66 allow custom classes to utilise the geometrymakerparse json method

[sid-mungale]

Initialise GeometryMaker with a custom class and it should be able to make geometry using it

New

3custom_component.py, 3custom_component.json: Demonstrate how to create a custom component

Updated

geometry_maker.GeometryMaker

[lukethehuman]

Very happy to see this support being added, though I think there may be a better way to do this that doesn't require passing in the names of custom classes explicitly. See comment for suggestion.

[lukethehuman]

I think we should also have a test for this functionality, the same or slightly simpler version of this example code should work well as a test.

[lukethehuman]

Also a minor gripe, but the filename is slightly hard to read, would suggest an underscore like 3_ (would want to do this for all your example files to be consistent ofc)

[sid-mungale]

Changed file names and added a test

[lukethehuman]

This works and might well be the best solution, but it could also be possible for us to avoid needing a keyword argument by taking advantage of the fact that the python contains a list of all the subclasses which inherit from a given base class in the current scope via: ComponentBase.__subclasses__()

Searching that list for the correct classname would discover both native hypnos subclasses and user-defined subclasses. Thus the user would only need to define their subclass and make sure it's in scope (e.g. import it) and hypnos could find it.

[lukethehuman]

The following function could support this:

def get_subclass_from_classname(classname) -> ComponentBase:
    """Return a ComponentBase subclass by name."""
    subclasses = ComponentBase.__subclasses__()
    for subclass in subclasses:
        if subclass.__name__ == classname:
            return subclass
    raise ValueError("No such class.")
```

[sid-mungale]

I was a bit paranoid about scopes as i think I'd originally tried something similar to this and it didn't discover every subclass, as well as feeling painful to call every time a class needs to be instantiated. Hence I opted to make a dictionary in #91 and append custom components manually. I could look into what could be causing the bug if this is a preferable way though?