Devise robust adapter principle and backend

[slobentanzer]

We should have at least one "default" adapter for the open targets pipeline output files in .parquet format that resembles our preferred way of processing this dataset. We should represent all relevant features of the dataset, and provide an adapter that is easily reused and understood by developer users. It should be broadly runnable on various operating systems.

Related to #17 and #18.

[kpto]

Not so sure this should be in version 0.4.0 milestone anymore, removing this from it for now.