Automatically convert stereo books to mono if too big for one CD

[egli]

Books are converted to mp3 using the DAISY Pipeline1 Audio Transformer and ultimately lame. Currently stereo books are converted as stereo and mono books as mono. This was done so that if a book had stereo we would retain the quality.

However this has the effect that the stereo productions often do not fit on one CD and will have to be split manually. Instead mdr2 should try to switch to mono automatically when calculating the ideal bitrate and no bitrate can be found for stereo