Draft: Highshelf Turbulence AM filter

Implementation of time-variant high-shelf filter for turbulence AM according to

[1] Pieren, Reto, et al. "Improving future low-noise aircraft technologies using experimental perception-based evaluation of synthetic flyovers". Science of the Total Environment 692 (2019): 68-81.

The missing piece to make this work properly, is low-pass filtering the random gain variable g(t) using a cut-off frequency of a few Hz.

Currently, the variable is updated every few audio blocks instead of actually being low-pass filtered. This can lead to sudden jumps in the resulting AM filter. In the course of implementing the low-pass filter for g(t), this should be changed back to updating g(t) for every audio block.