External models

[beasteers]

This lets you easily define a crema model that lives outside of the core repo, which I imagine most models are at some point before they get merged in. This has no effect on the default behavior and only matters for subclasses that modify these attributes. So for example:

import crema
from .custom_layer import CustomLayer

class MyModel(crema.models.base.CremaModel):
    name = 'my_model'
    model_root = '~/.crema/models'
    custom_objects = {
        'CustomLayer': CustomLayer
    }

And voila - an externally defined model at ~/.crema/models/my_model can be used as a crema model with its own custom layers.

[beasteers]

The test failure happens when installing llvmlite on python 3.5. it looks like it's an issue upstream where some package is not supporting 3.5.