decomposition of model_interfaces.py

[hstojic]

Hello,

here is a draft for decomposing the model_interfaces (now also renamed to interfaces)

some remarks:

• I didn't move optimizers yet, that would be the next step - it needs some further rethinking and changes in some classes as default gpflow optimizer is spreaded elsewhere
• I haven't touched tests, after agreeing on final form I can do that part as well
• it would be great to eliminate gpflow imports anywhere else from the code, isolate it to models_gpflow
• this PR will make the toolbox incompatible with previous versions, so perhaps this is something to implement for trieste 1.x

[hstojic]

Finally updated all this. Merged in latest develop, moved around all the tests, eliminated gpflow from other parts of the code.

I also attempted to fix config.py by creaating a parent class and creating model type specific subclass, but there typing turned out to be messy and mypy complains - I would appreciate some help in this department :) (@joelberkeley?)

I haven't touched optimizers yet, not crucial for now as everything can work without that part, so I left it for another PR.

[uri-granta]

This looks great. I'll take a look at the typing next.

Note that it'll result in a small merge conflict for #320 (cc @sebastianober)

Also probably worth building the docs manually using tox to see that they still look ok.

[uri-granta]

Looking in more detail at the typing failures, there are quite a few issues. Happy to discuss.

[hstojic]

All fixed now and merged in latest develop.

I returned get_kernel method to ProbabilisticModel and we'll address that in another PR.