Add more WDF adaptors

ParallelTwoportAdaptor
SeriesTwoportAdaptor
ParallelThreeportAdaptor
SeriesThreeportAdaptor
ReflectionFreeParallelThreeportAdaptor
ReflectionFreeSeriesThreeportAdaptor
Move all WDF operations (and tests) to wdf_operations.py

Four-port adaptors are also interesting, but there are two options for "reflection-free".