FILTERS: Fix dumb typo in Chamberlin sandbox

Actually this does not change anything in the conclusion of the
"improved vs raw version" debate.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
FILTERS: Add a 1st order Pole-zero low pass filter prototype

Also added a vectorized version of the processing method.
Fixes #16.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
FILTERS: Add a C++ version of the 1st order Poe-Zero lowpass filter

A vectorized version, a bit faster than the base one, was also added.
Part of #17.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
FILTERS: Add basic tests for 1st order Pole-Zero lowpass

These pass in both scalar and vectorized versions.
Fixes #17.

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