Add node.Splitter()

[lantz]

Splitter() is a simple splitter element that currently does not model coupling loss (although the split percents may be adjusted to model it as desired.)

Note that Splitter() copies the signal to the split port if needed, creating a new signal that originates at the splitter. We do this in case this signal
needs to propagate through one of the elements that the original signal propagates through.

• add tests/splittertest.py

• also remove dodgy intermediate signal state storage in OpticalSignal

• also make assoc_loc_in/out more reliable by requiring at least one parameter

• also remove unused include_optical_signal_in_roadm and references to it