Externalize pipeline architecture

[Callidon]

Implements the functionalities discusses in #15
WARNING: this PR introduces major changes to almost every class in the framework, so it will likely break all existing forks.

The changes are:

• All the pipeline-related operations are now abstracted using PipelineEngine.
• Subclasses of PipelineEngine are used to execute every operation in the main query engine, which removes the strong coupling to rxjs.
• RxjsPipeline is a subclass of PipelineEngine which implements the pipeline operations using rxjs. It corresponds to the engine behaviour before this PR. RxjsPipeline is used as the default pipeline implementation.
• VectorPiepline is a subclass of PipelineEngine which follows the vectorization paradigm: all intermediate results are materialized in main memory. This pipeline impl. is more efficient CPU-wise, but consume more memory.
• Added a configurable test suite to test all subclasses of PipelineEngine.
• Update the README with a short tutorial on these new functionalities.