OpCompressedCache: fixed comment.
[OpLabelVolume] updated documentation, introduced new tests for threading and Output/CachedOutput behaviour
[OpLabelVolume] removed duplicate code
[OpLabelVolume] added test for correct blocking
Added scikitClassifiers.py, which contains classifier operators with swappable implementation.
Defined classifier interface and replaced old RF classifierOperators with generic versions.
classifiers: Refactored again.  Distinction between 'classifier' abc and 'classifier factory' abc.
classifier operators: classifier factory is configured as a slot, not a constructor arg
classifiers: Added serialization
classifiers: Added parallel RF classifier (the original classifier implementation)
Merge pull request #135 from burgerdev/OpLabelVolume-remove-code-duplication

[OpLabelVolume] reduce code duplication
OpCompressedCache: fixed comment.
Merge branch 'scikit-classifier'
OpClassifierPredict: Added PredictionMask slot, which can be used to skip prediction if the entire roi is masked out.
OpClassifierPredict: Slight optimization for masked case.
classifiers: Corrected ABC for serialization functions.
Removed accidentally committed file.
classifiers: Renamed classifier ABCs and defined a new ABC for "pixelwise" classifiers.
Slot, Operator: Refinement to cleanUp behavior: Don't automatically call resize(0) on disconnect during cleanUp.
(Otherwise, this causes problems for remaining OperatorWrapper connections.)
classifiers:
- Added example pixel-wise classifier
- classifier training and prediction operators can handle either type of classifier and classifier factory.