#59 convolutionHelper \w kernel2D + boundary Zero, Mirror buggy

[Peter Doerfler]

In convolutionHelper and thus in convolution,
erosion, dilation, and skeleton the boundaryType
Zero and Mirror are buggy if a kernel2D is used.

The Zero problem does not affect convolution.
However for morphological operations the 0 outside
source is not used in the calculation.

Mirror does not duplicate the first/last element within
the src.

[Peter Doerfler]

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fixed for Zero.

Mirror is much more work and rarely used. Postponing.