Rework maths operators

• add unit test for mln/core/image/view/maths operators
• add missing maths operators: dot, cross, min, max, l2dist_sqr, l2dist, l2norm_sqr, lpnorm, lpdist
• add missing documentation
• investigate gcc compile error
• add mln::channel, mln::red, mln::green, mln::blue (migrate code + doc)

fixes #52 (closed) #53 (closed)