Extend image blending module

[aleju]

With this patch the available augmenters for
alpha-blending of images are significantly extended.
There are now new blending augmenters available to
alpha-blend acoording to:

• Some randomly chosen colors. (BlendAlphaSomeColors)
• Linear gradients. (BlendAlphaHorizontalLinearGradient,
  BlendAlphaVerticalLinearGradient)
• Regular grids and checkerboard patterns. (BlendAlphaRegularGrid,
  BlendAlphaCheckerboard)
• Only at locations that overlap with specific segmentation class
  IDs (or the inverse of that). (BlendAlphaSegMapClassIds)
• Only within bounding boxes with specific labels (or the inverse
  of that). (BlendAlphaBoundingBoxes)

This allows to e.g. randomly remove some colors while leaving
other colors unchanged (BlendAlphaSomeColors(Grayscale(1.0))),
to change the color of some objects
(BlendAlphaSegMapClassIds(AddToHue((-256, 256)))), to add
cloud-patterns only to the top of images
(BlendAlphaVerticalLinearGradient(Clouds())) or to apply
augmenters in some coarse rectangular areas (e.g.
BlendAlphaRegularGrid(Multiply(0.0)) to achieve a similar
effect to CoarseDropout or
BlendAlphaRegularGrid(AveragePooling(8)) to pool in equally
coarse image sub-regions).

Other mask-based alpha blending techniques can be achieved by
subclassing IBatchwiseMaskGenerator and providing an
instance of such a class to BlendAlphaMask.

This patch also changes the naming of the blending augmenters
as follows:

• Alpha -> BlendAlpha
• AlphaElementwise -> BlendAlphaElementwise
• SimplexNoiseAlpha -> BlendAlphaSimplexNoise
• FrequencyNoiseAlpha -> BlendAlphaFrequencyNoise
  The old names are now deprecated.
  Furthermore, the parameters first and second, which were
  used by all blending augmenters, have now the names foreground
  and background.

List of changes:

• Add imgaug.augmenters.blend.BlendAlphaMask, which uses
  a mask generator instance to generate per batch alpha masks and
  then alpha-blends using these masks.
• Add imgaug.augmenters.blend.BlendAlphaSomeColors.
• Add imgaug.augmenters.blend.BlendAlphaHorizontalLinearGradient.
• Add imgaug.augmenters.blend.BlendAlphaVerticalLinearGradient.
• Add imgaug.augmenters.blend.BlendAlphaRegularGrid.
• Add imgaug.augmenters.blend.BlendAlphaCheckerboard.
• Add imgaug.augmenters.blend.BlendAlphaSegMapClassIds.
• Add imgaug.augmenters.blend.BlendAlphaBoundingBoxes.
• Add imgaug.augmenters.blend.IBatchwiseMaskGenerator,
  an interface for classes generating masks on a batch-by-batch
  basis.
• Add imgaug.augmenters.blend.StochasticParameterMaskGen,
  a helper to generate masks from StochasticParameter instances.
• Add imgaug.augmenters.blend.SomeColorsMaskGen, a generator
  that produces masks marking randomly chosen colors in images.
• Add imgaug.augmenters.blend.HorizontalLinearGradientMaskGen,
  a linear gradient mask generator.
• Add imgaug.augmenters.blend.VerticalLinearGradientMaskGen,
  a linear gradient mask generator.
• Add imgaug.augmenters.blend.RegularGridMaskGen,
  a checkerboard-like mask generator where every grid cell has
  a random alpha value.
• Add imgaug.augmenters.blend.CheckerboardMaskGen,
  a checkerboard-like mask generator where every grid cell has
  the opposite alpha value of its 4-neighbours.
• Add imgaug.augmenters.blend.SegMapClassIdsMaskGen, a
  segmentation map-based mask generator.
• Add imgaug.augmenters.blend.BoundingBoxesMaskGen, a bounding
  box-based mask generator.
• Add imgaug.augmenters.blend.InvertMaskGen, an mask generator
  that inverts masks produces by child generators.
• Change imgaug.parameters.SimplexNoise and
  imgaug.parameters.FrequencyNoise to also accept (H, W, C)
  sampling shapes, instead of only (H, W).
• Refactor AlphaElementwise to be a wrapper around
  BlendAlphaMask.
• Rename Alpha to BlendAlpha.
  Alpha is now deprecated.
• Rename AlphaElementwise to BlendAlphaElementwise.
  AlphaElementwise is now deprecated.
• Rename SimplexNoiseAlpha to BlendAlphaSimplexNoise.
  SimplexNoiseAlpha is now deprecated.
• Rename FrequencyNoiseAlpha to BlendAlphaFrequencyNoise.
  FrequencyNoiseAlpha is now deprecated.
• Rename arguments first and second to foreground and
  background in BlendAlpha, BlendAlphaElementwise,
  BlendAlphaSimplexNoise and BlendAlphaFrequencyNoise.
• Change imgaug.parameters.handle_categorical_string_param()
  to allow parameter valid_values to be None.
• Fix a wrong error message in
  imgaug.augmenters.color.change_colorspace_().
• Remove imgaug.augmenters.color.GrayscaleColorwise
  and imgaug.augmenters.color.RemoveSaturationColorwise
  as these are now covered by BlendAlphaSomeColors.

[codecov-io]

Codecov Report

Merging #556 into master will increase coverage by 0.04%.
The diff coverage is 95.24%.

@@            Coverage Diff            @@
##           master    #556      +/-   ##
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+ Coverage   96.45%   96.5%   +0.04%     
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  Files          40      40              
  Lines       14230   14496     +266     
=========================================
+ Hits        13725   13988     +263     
- Misses        505     508       +3