Add option to spatialise using VBAP to arbitrary loudspeaker arrays.

[marc1701]

Add option to spatialise using VBAP rather than Ambisonics.

Steps:

• Store loudspeaker positions (see ambidecstrous) as array of cartesian co-ordinates (unit sphere).
• Also convert desired DOA of current frame to cartesian co-ordinates, and work out angular distance to loudspeaker positions to find nearest three to use as active triangle.
• Calculate gains g according to Equation 18 from Pulkki's VBAP paper.

[marc1701]

Implemented DBAP instead