Default structuring element (selem)

Functions based on erosion or dilation no longer require the user to specify a structuring element. The default structuring element is a cross-shape (connectivity=1). The number of dimensions in the image determines the dimentionality of the "hyper-cross" structuring element (ready for N-dimensional images).

Tests have been added to compare the results of the function called with an explicit structuring element with the function called without specifying the "selem".
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