tiny-plot: make class group colors consistent across samples in scatter_dge_class

[AlexTate]

Currently, classes are sorted by the number of associated features before plotting for sample_avg_scatter_dge_class plots. This ensures that samples with fewer associated features are plotted on top of more abundant classes. However, this sorting happens on a per-sample basis, and class colors are currently determined by this sorted order. This can lead to variation in the colors assigned to classes in each sample.

This plot type will be updated so that class colors are consistent across samples.