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

[khaeru]

In #223, there was a discussion (#223 (comment)) 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.

[khaeru]

Postponing to after v3.0.

[khaeru]

No one was assigned to do this by the feature freeze for v3.2.0, so we postpone to v3.3 (or later).

[khaeru]

No one was assigned to do this by the feature freeze for v3.3.0, so we postpone to v3.4 (or later).

[LauWien]

No one was assigned to do this for v3.4.0, so we postpone to v3.5 (or later).

[LauWien]

No one was assigned to do this by the feature freeze for v3.5.0, so we postpone to v3.6 (or later).

[khaeru]

Removing the milestone entirely. It can be re-added if/when the team decides to schedule/assign the work.