Show alteration frequencies for groups in OncoPrints

[schultzn]

OncoPrints currently support the visual separation of groups, as defined by clinical data elements in annotation tracks (via the "Show gaps" option in the Annotation track menu), see this example:
https://bit.ly/3IaS8YZ

A missing feature is to indicate the alteration frequencies of the individual groups, like in this example from Chatila et al. [https://doi.org/10.1038/s41467-022-35592-9]:

We could add a new menu item in the settings menu for the track, e.g.: "Show alteration frequencies of each group"

Added as GSoC issue as well: cBioPortal/GSoC#99