SampleUtil Enhancements

Enhancements and extensions of SampleUtil. Some of the new functions are currently not used, but will be when the new SoundSource API follows. I adopted the naming "widen"/"narrow" for the conversion of signals between a different number of channels: mono --"widen"--> stereo <--"narrow"-- multi-channel

Some minor issues are fixed in separate commits.

CSAMPLE_MIN and CSAMPLE_MAX might be ambiguous
What about reusing CSAMPLE_PEAK and -CSAMPLE_PEAK here? 
In general Samples > 1.0  are allowed as long we are inside Mixxx only.