Feature: directivity support

This MR add directivity support to DOVELET. This means both source as well as receiver directivities. As such, with this MR, a Directivity spatialization was added, which should be able to be used to create binaural synthesis.

The directivity settings here will overwrite any information from PELICAN (which currently does not supply it anyway).

Please note, that this is not 100% polished but workable.