Separate files for new color model

This PR introduces two new files, earlyvision_color.py and gcal_color.py. In earlyvision_color.py, EarlyVisionModel is extended to include color related code for the visual pathway. gcal_color.py is nearly an exact copy of gcal.py, except that some code related to color_scale and measurement for color is added. This is a temporary solution to replicate the results of fischer:ms14. As soon as a modular model structure is possible, most of the duplication will be able to be removed.