Document background classes

Users of classifiers (DTW or kNN) want to know when there is no gesture that matches what has been trained. We need to document how the possible use of a "background" class, or rejecting bad matches by thresholding.

@francisco Does this make sense?

moved to RAPID-MIX_API#4

It does. This was one of the important findings from eNTERFACES17. We need to convey the importance of background class, it's nowhere in the documentation.

This is a closed issue in RapidLib. You want the one in RAPID-MIX_API, linked above.

added UCD label