QC copy number data

https://github.com/greenelab/mutation-fn/pull/3#discussion_r785038890

We should generally take a closer look at this data - e.g. what is the distribution of copy number counts across samples? Does this look like what we would expect given previously observed trends in various cancer types?

Start with the linked pull-request discussion and inspect the copy number data used there. Characterize the distribution of copy number counts across samples and compare it with expected trends across cancer types; the work is complete when these QC findings are recorded and any data-quality concern is resolved.

Written by the indexing model from the issue text.