DESCRIPTORS: Now all descriptors must provide their metadata

This is more flexible and cleaner for each descriptor to have to
implement such a method, with the associated metadata.
The only downside is that this metadata retrieval is no longer static.
Fixes #24.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
ALGORITHMS: Make an actual test on spectrogram output

This can now be made thanks to the new metadata retrieval system.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
MANAGER: Add all build files and directory for the new Manager class

Part of #27.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
MANAGER: Created the descriptor manager class

Fixes #27.

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