Cohort Membership: Resolve design inconsistents

Break the following tasks into subtasks or other tickets that are ready for work:

1. Make these methods behave like typical Java Collection of membership (excluding voided memberships):

• contains(Integer patientId) - true if any non-voided membership for patient
• size() - total number of non-voided memberships
• isEmpty() - true if size() == 0

2. Refactor getMemberships() to exclude voided memberships.

3. Add methods for interacting with active memberships directly at Cohort level:

• hasActiveMembership(Integer patientId) - true if has active membership for patient
• activeMembershipSize() - number of active memberships
• hasNoActiveMemberships() - true if activeMembershipSize() == 0

4. Implement CohortMembership.equals() to return true if patient, start, end end dates match

Will not be covered by this ticket:

• Is "voided" really necessary for CohortMemberships?
• Unclear what the union, intersect and subtract utility method should do; ie, do they take the dates into account?  (I believe right now they don't.)