change the way expression file classes are coloured.

[risserlin]

currently the class file is dependent on the phenotypes in each dataset. could we uncouple that connection and just define the colours highlighted above the expression file for any class file that has more than 2 classes automatically. (any colours other than red and blue. maybe one colour with decreasing saturation)

special case is when there are only two classes as I think red/blue for na_pos/na_neg is important.

It would be nice to be able to associate the dataset with the columns of expression. Maybe using the na_pos phenotype value matching the class file or something.