Support easy Cohort creation

[tavinathanson]

Making a Cohort with appropriate Patient and Sample objects can be a bit of a pain. It's a necessary pain when reading in data from multiple sources, as we often do. It shouldn't be necessary if all relevant data already lives in one DataFrame; in that case, we could have e.g. Cohort.from_dataframe(df, id="patient_id", ...) to build the Cohort.