Make code more flexible & publish interfaces to be able to implement your own components

[alex-movila]

Make the code more modular to be able to implement your own reader , retriever etc. Document the interfaces.

[fmikaelian]

We were actually discussing about this idea with @andrelmfarias a few days ago :-)

It would be great to be able to define a cdQA pipeline like this:

QAPipeline(retriever='BM25', reader='XLNet')

where the reader and retriever would be class objects with a pre-defined convention, like the sklearn BaseEstimator.