Warm start from previously solved scenario

This issue would address potential solve speedups (e.g., I am seeing week-long solve times at the moment for some scenarios).

The idea is straightforward - for a scenario which has a solution, provide this existing solution as a "warm start" (i.e., providing a starting basis) for a new scenario run.

In principle, all components for this are available, but need to be connected properly. The steps would be as follows:

1. declare base scenario to use
2. read base scenario solution
3. construct warm-start basis from solution
4. provide new basis to solver (which may require writing a file to disk)

This could be one way to greatly speed up the existing global message experimental set up (which requires multiple sequential solves)

cc @OFR-IIASA @khaeru

The issue names no files or tests; begin by tracing how a solved scenario is declared and how its solution reaches the solver. Map the four listed steps, then confirm completion when a new scenario accepts the prior solution as a warm start and solve performance can be compared.

Written by the indexing model from the issue text.