Lissom color fixes for Blue-RedGreen LGN sheets

This pull request consists of two small bugfixes related to the Blue-RedGreen LGN sheets.
1: So far a fixed value of 4.7 was used as strength from the retina sheets to the Blue-RedGreen LGN sheets. However, this value is only suitable for natural image datasets, and only if speed=0. Instead, strength_scale should be used which depends on
  * Whether it is a natural image dataset or Gaussian input
  * How fast patterns are moving

2: The red and green retina sheets were connected to the Blue-RedGreen LGN sheets using center weights. Instead, they should be connected using surround weights. Only the blue retina sheets should use center weights (as they do correctly).