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".
Haha weird, it's still doing it because you added four spaces...!