Migrate reserve_margin()

[khaeru]

In a review of #207, @glatterf42 noted that .model.workflow.solve()—that is, a standard "solve" workflow step for MESSAGEix-GLOBIOM workflows—uses a function message_data.scenario_generation.reserve_margin.res_marg.main() that is part of the closed-source message_data package.

For the moment, this step is optional and not tested, so that it lives in message_data is not a critical issue. But it limited the usability of the calling function to users who have access to message_data.

To resolve:

1. Find a suitable location for this function in message_ix_models.
2. Update its documentation.
3. Add tests.
4. Update usage in message_ix_models.
5. Deprecate the version in message_data.