Design consideration: move tolerance into the optimizer

[alphaville]

Currently we store the tolerance in the cache, but it seems to me that storing it in the optimizer would make more sense. For example, we have a setter method in panoc_optimizer which is as follows:

pub fn with_tolerance(
        mut self,
        tolerance: f64,
    ) -> Self {
        assert!(tolerance > 0.0, "tolerance must be larger than 0");
        self.panoc_engine.cache.tolerance = tolerance;
        self
    }

and it sets the tolerance in the cache object.

I am currently addressing issue #70 where we will need to introduce another tolerance value (namely, the akkt_tolerance). I think it makes more sense to have these tolerances be part of the optimizer.

[alphaville]

Never mind - I realised why we did it like this.