Clarify add_macro() behaviour on a non-'clean' Scenario

In #223, there was a discussion (https://github.com/iiasa/message_ix/pull/223#discussion_r312356449) of how Scenario.add_macro should behave when the target Scenario already contains MACRO-related items or data; i.e. should it:
- refuse to proceed and raise an Exception,
- overwrite the existing content, or
- something else?

At minimum, a clear description of the behaviour is needed before this feature can be released with message_ix 3.0. Ideally, more thorough checks and informative messages would help users avoid errors.

Start at the Scenario.add_macro entry point and review the discussion linked from issue #223. Decide and document how the method handles existing MACRO-related content, then define the checks and informative messages needed to make that behavior clear.

Written by the indexing model from the issue text.