helical: pylint errors

pylint gives the following errors:

************* Module nabu.pipeline.helical.filtering
nabu/pipeline/helical/filtering.py:102:12: E1121: Too many positional arguments for method call (too-many-function-args)
************* Module nabu.pipeline.helical.span_strategy
nabu/pipeline/helical/span_strategy.py:81:52: E1130: bad operand type for unary -: NoneType (invalid-unary-operand-type)

Related: !223 (merged)