COMMON: Add an include for using std::size_t definition

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
DESCRIPTORS: Descriptor interface no longer using std::array

Now using a classical pointer-and-length pattern. This allow different
array size as argument.

Part of #26.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
DESCRIPTORS: Update to new descriptors interface

Part of #26.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
ALGORITHMS: Better consistency between dft algorithms

Now they both take the Dft length at construction, not being able
to change it later.
Part of #26.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
ALGORITHMS: Make algorithms derive from descriptor interface

This makes sense for the Dft and spectrogram algorithms.
ASC was updated accordingly, as well as all tests.
Fixes #26.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
ALGORITHMS: Make the overlap a member of Spectrogram algorithm

This is still a static member, but this is better.

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