Implement RequiredInterfaces

[stelmo]

@exaexa do you like this? See here for the docs of this pkg: https://seelengrab.github.io/RequiredInterfaces.jl/stable/examples/basic.html

I can include some tests that make a fake module, implement the required interface, and check if it satisfies the requirements, but this feels a little circular...

[exaexa]

I guess it's OK, not much need to test stuff here... What does the error look like if you derive an incomplete model?

[stelmo]

It throws errors like this: https://seelengrab.github.io/RequiredInterfaces.jl/stable/examples/testing.html

[stelmo]

btw, it should be Nothing since you specify the type in the assert

[stelmo]

anyways, I would merge this, and edit as necessary