Split morphological demos into smaller pieces

[johnnychen94]

The current two demos added by #202 and #203 are perhaps too coarse for users to quickly get an overview from the cover images, thus I think it makes sense to have more smaller ones for:

• open and close
• erosion and dilation
• tophat and bothat
• morphological gradient and morphological laplace
• convex hull
• image filling
• image thining
• clear boarder

Because we use morphology_test_512 for the purpose, we can cut to the chase and only describe the definition, and only show the result.