New notebook: Theory of spatially band-limited matched filter beamformer

[narahahn]

This notebook describes the theory of matched filter beamformer for an open sphere.

• matched filter beamformer in the frequency domain
• matched filter beamformer in the time domain
• derivation of the beam pattern and the impulse responses

[narahahn]

This branch is growing in a direction that does not fit very well to sfa-numpy. I close this PR and continue the work in a new repository: https://github.com/spatialaudio/time-domain-modal-processing