Implement c_system_coverage

[mikix]

From a researcher point of view, I might want to know "If I'm going to be analyzing Conditions, which systems do I need to look at to get the best coverage?"

(e.g. "snomed get me 90% of conditions, but snomed+icd10 get me 100%, so those two are all I need to focus on in my study")

This is c_system_coverage

I think from a data-metrics jinja perspective, this would look similarly to c_resources_per_pt as a line-level characterization metric (i.e. not a CUBE)