Add Slopes script

[chrisib]

Adds a new script that generates gates & CV signals based on the slope of an incoming LFO, envelope, or other signal.

Connect an analogue signal to ain. cv1-3 output gates depending on whether the slope of the incoming signal is positive, negative, or steady. cv4-6 output CV signals based on the steepness of the slope (e.g. a faster LFO will change faster, producing steeper slopes, resulting in stronger output CV signals). For best results ain should have a varying slope (e.g. triangle or sine); a sawtooth wave will result in pretty boring outputs as the slope is consistently the same.

k1 acts as a de-noise adjustment. Increasing it will result in smoother outputs, but may make the script less responsive to rapid changes in slope.

k2 acts as an output attenuator for cv4-6.

[chrisib]

Note: CI failure is due to the renamed experimental.math -> experimental.math_extras change in #371.