Plot mean expression (over selected genes) in a separate bubble plot

[bhavyaac]

As of v1.1 (PR #12), the mean expression is plotted using the same functions used for the gens above it, as if it were a gene named "MEAN". For a cleaner approach that is easier to build upon (e.g. to add other statistics such as ssGSEA score between plotted genes, mentioned in issue #3), this row should instead be plotted as a separate bubble plot.
This also means that a separate colour scheme & corresponding legend can be defined for this plot, to enable a clearer representation.