MANAGER: Add a "read-only" descriptor retriever

In this version, a const pointer to the computed data is given.
This makes it more efficient while the computed data is kept by
the manager.
Part of #29.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
ALGORITHMS: Move Dft call out of spectrogram, into the manager

Part of #29.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
ALGORITHMS: Add 2 new algorithms for spectrum power computation

Part of #29 - those are required by ASC and ASS.
Note that these algorithms are identical - see related issue #31
for future enhancement tracking.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
MANAGER: Integrate new algorithms into the manager

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
MANAGER: Minor code renaming/cleaning

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
DESCRIPTORS: Move all heavy computation from descriptors into the manager

Fixes #29.

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