Separate microphones and beamformers objects

[fakufaku]

Currently, beamformers are a sub-class of microphone array objects. Ideally, the beamformers should have a structure more similar to the DOA or adaptive filters objects. They could be a subclass of STFT processing too.
It would be good that all the beamforming algorithms are also defined as sub-classes of a parent abstract class. This way, new algorithms could be easily added.

[duembgen]

@fakufaku I am interested in this enhancement, because I am thinking about doing some experiments with MVDR, for example. As soon as I start working on it I can share here if I can do this separation, if you still think it would be interesting.