Where to place code implementing new models

Note that different packages can implement models having the same name without causing conflicts, although an MLJ user cannot simultaneously load two such models.

There are two options for making a new model implementation available to all MLJ users:

1. Native implementations (preferred option). The implementation code lives in the same package that contains the learning algorithms implementing the interface. An example is EvoTrees.jl. In this case, it is sufficient to open an issue at MLJ requesting the package to be registered with MLJ. Registering a package allows the MLJ user to access its models' metadata and to selectively load them.

2. Separate interface package. Implementation code lives in a separate interface package, which has the algorithm-providing package as a dependency. See the template repository MLJExampleInterface.jl.

Additionally, one needs to ensure that the implementation code defines the package_name and load_path model traits appropriately, so that MLJ's @load macro can find the necessary code (see MLJModels/src for examples).