rename FakeSeriesSink -> NullSeriesSink

docs/api/sinks/sinks.md

src/sgnts/sinks/__init__.py

tests/test_adder_align.py

tests/test_align.py

tests/test_gate.py

tests/test_lloid.py

tests/test_mult.py

tests/test_multi_sink.py

tests/test_realtime.py

tests/test_resource_source.py

tests/test_segmentsrc.py

tests/test_sumindex.py

tests/test_threshold.py

tests/test_tsbuffer.py