Rank filter context

I did some modifications. Can you give me feedback whether style is ok, then I try to do the rest tomorrow.

Changes made so far:

1. filter_rank module is split in 4 different modules:

   1. filter_neighbourhood
   2. filter_rank
   3. filter_morphological
   4. local_background_subtraction

2. Erosion and dilation are defined as "morphological operations", which build on top of "rank filters"

3. rank filters module

   1. changed concept map
   2. add requirements, motivation, and learning objectives

4. morphological filters module

   1. change concept map
   2. formula explained in concept map

Context of local background block and neighbourhood filters block not changed yet.