MODULATORS: Added a modulators DSP part

This includes all required build files, a common header/source couple,
a factor header/source couple, and a common interface for all future
envelop generators.
It also involvves modifying the existing build files in order to take
this new part into account.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
MODULATORS: Added a C++ implementation of an ADSD modulator

Based on the one prototyped in Python.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
SYNTH: Added a "voltage-controlled amplifier" object

By analogy with VCF/VCO objects, this wraps an internal envelop generator,
and allow paramters management as well as appliyng the final envelop on
any given signal.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
SCRIPTS: Cleaned prototype ADSD implementation

Useless variables were shaved off.
Note that this does not imply that this version is the optimized one.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>