ENH: control amplitude envelope of the coupled waveform

[ctrltz]

This PR:

• introduces control over amplitude envelope for Hilbert-based coupling functions (for now, using either the envelope of input waveform or the envelope of a randomly generated waveform)
• fixes tests for the von Mises coupling function - adds a fixed random state for stability, set both lower and upper bound for the expected coupling value. Close Tests for the von Mises coupling are unstable #32

And fixed the bug, the results of select_kappa example seem fine again:

[astudenova]

Everything seems to work well. ദ്ദി