adding multi-instance-tuner

[ElFosco]

Adding multi-instance function for both ParameterTuner and GridSearchTuner.
This pull request fixes Issue #751.

[tias]

Hi Marco,

Seems like you are fixing two things in one pull request:

• Timer fix, great, this could go in easily if in a separate PR

• Multi-instance tuning, also great, however, its design is not sustainable because it copies code (so double subtle maintenance), the tune_list also silently assumes that 'model' can be a list.

The latter needs to be reworked. A clean solution that could work with any tuner without having to duplicate its code, is to create a new class, something like 'MultiSolver' that mimics being a single solver for all instances. In any tuner, if 'model' is a list, you would not solver = SolverLookup.get(self.solvername, mdl) but you would solver = MultiSolver(self.solvername, models). This multisolver, when .solve(time_limit=...) is called on it, it will create and call a solver for each instance, respecting the global time limit.

This would work for any tuner and would be easy to maintain, do you agree?

[ElFosco]

Thank you Tias for the comments. I fully agree, I will work on it later this afternoon.

[ElFosco]

Sorry, apparently there is a bug that I just noticed now. I am going to fix it and should be ready