Clean up comparator code in `behavior_ophys_nwb_api`

[kschelonka]

Some custom comparators used in tests are defined in allensdk.brain_observatory.behavior.behavior_ophys_api.behavior_ophys_nwb_api:

equals
compare_fields

These methods are supposed to apply to any ophys API, so they shouldn't be defined in behavior_ophys_nwb_api anyway.

We should:

• Remove these methods from behavior_ophys_nwb_api
• Decide whether or not we care about equality between two Session or API objects
• If we do care about equality, implement __eq__ in BehaviorOphysApiBase and BehaviorOphysSession
• Update any tests that are affected