Reduce code duplication in GaussianBlur
Reduce code duplication in GaussianBlur

This patch decreases code duplication in the
parameter parsing of
augmenters.blur.GaussianBlur by using the
parameter handling functions in parameters.py.

It also enables the sigma parameter to
convert a list of values to Choice.

aleju/ImageAugmenter,aleju/imgaug,aleju/imgaug