New strel methods to calculate size of padding (fixes for even sized nhood). * imerode.cc: make use of new pre_pad and post_pad methods, fixing the bugs when doing dilation with even sized nhood. Use the octave::image namespace. Mark previous xtest as normal assert since it's fixed now. * strel.h, strel.cc: new pre_pad and post_pad methods. Add comments on rationale of the class and how origin coordinates are treated.