Output a log/plot of MACRO calibration progress

Related to #719, @OFR-IIASA pointed to files like [ssp3_60_3_51_00_macro_iteration.pdf](https://github.com/iiasa/message_ix/files/12183978/ssp3_60_3_51_00_macro_iteration.pdf) (from P:/ene.model/SSP_V3/macro_runs6/log/).

This is an output file produced by some older (R?) MACRO calibration tool that shows how `demand` values change during iterations in the MACRO calibration process.

The current `message_ix.macro` could be enhanced to generate similar output as an aid to checking the calibration has produced intended results.

Start with the current `message_ix.macro` calibration process and compare its outputs with the linked `ssp3_60_3_51_00_macro_iteration.pdf` example. Determine how to expose a log or plot of changing `demand` values during iterations; done means the output helps check that calibration produced the intended results.

Written by the indexing model from the issue text.