Cleaning of `cplex.opt` causes asyncronous issues with multiple runs

[gidden]

I have experienced silent errors when performing multiple message_ix.Scenario.solve()s simultaneously on the same computer. As best I can tell, these occur when I employ the solve_options kwarg, which forces a writing to disk of the cplex.opt file.

Upon completion, I believe this file is then garbage collected. However, other solves occuring simultaneously expect for this file to be present.

As a short solution, I would suggest we simply do not erase the file. A better solution would employ asyncronous file manipulation.

[gidden]

As I look through the code base, it seems like this was a known issue at the time, specifically citing this comment.

[khaeru]

This may also be a duplicate of #250.

[gidden]

closed via #657