Update handling of discount/interest rates in M-M link

[khaeru]

During #223, which added MACRO calibration for MESSAGE-only Scenarios, some issues with usage of the (time-varying?) interestrate and (node-specific) drate (discount rate) parameters were identified. See:

• changed drate parameter in MACRO to MESSAGE interestrate in 3 remaining places gidden/message_ix#5 changing from drate to interestrate, and
• gidden/message_ix#7 reverting this change.

To resolve this:

1. Consider and decide which parameters should be used where.
2. Clearly describe this logic for both users and future developers of MESSAGE.
3. Per (1) add tests and/or checks for any imaginable edge/corner cases where certain parameter values might produce unexpected behaviour of the model.