Return information on the optimiser accuracy

[njtierney]

The solver accuracy should be returned

[jeffreyhanson]

I would be interested in hearing how you end up achieving this. It's kind of an afterthought in prioritizr - we just tack this information on "solution" object using attributes since we wanted to return the "solution object" in the same class as the input data to make it easy for users interact with solutions (e.g. as opposed to creating a custom S4 class that contains the solution but a bunch of other information).