Add support for Cohorts

[rcdilorenzo]

Examples:

phc.Patient.get_data_frame(cohort_name="MyCohort")

phc.Cohort.get_patient_ids(name="MyCohort")

(Note: Cohorts can be static or dynamic.)

[rcdilorenzo]

Because we'll need to handle dynamic cohorts, we should also build a module that can perform the subject search:

phc.Patient.search({
     'domain': 'filter',
     'target': 'patient',
     'where': {'patient': {'and': [{'observations': {'code': [{'operator': 'eq',
           'target': '85339-0',
           'value': 'negative'}]}},
       {'observations': {'code': [{'operator': 'eq',
           'target': '49683-6',
           'value': 'negative'}]}},
       {'observations': {'code': [{'operator': 'eq',
           'target': '85337-4',
           'value': 'negative'}]}}]}}
})