Resolve "[characterization] Background noise mixing for OSBA does not work correctly"

Related to #109 (closed)

"Need at least one array to stack" error was occurring due to the background noise preprocessing.

Background noise with metadata was untested previously and a bug in metadata handling meant that the object positions for the preprocessed (loudness adjusted) background noise were not set from the input file.

Even though the audio is zero and positions are not used, if the object position is None or an empty list [] the binaural rendering returns an empty list, leading to a crash.

This MR fixes the issue, along with bringing an optimization to speed up a bottleneck for preprocessing; running the ITU filter executable in parallel. For high channel counts such as OSBA, the sequential processing was not optimal.