Change from Function et al. interfaces + magic to input functions struct about optimize

[btracey]

This should have

type Bound struct{Min float64; Max float64}

and

type LinearConstraint struct{
    A *mat64.Dense
    b []float64
}

instead of the anonymous types.

[btracey]

This currently has a problem in that there is no way to communicate how many non-linear constraints there are. We could have a NumConstraints(), but that would be unfortunate. Maybe just mandate that Constraints has to allow nil arguments the first time.

[hypoactiv]

I took an initial stab at this on my fork at https://github.com/hypoactiv/optimize today. Comments and criticism are welcome. A few notes on my changes:

• I didn't yet do anything to update the testing system to the new style, so 'go test' won't run.
• I removed the FunctionInfo struct that was originally being passed around, since it seemed redundant after introducing the Problem struct. Instead of using funcInfo.IsGradient, etc, I check if p.Grad != nil.
• The Problem struct currently doesn't have any fields for constraints.

[btracey]

Would you mind making a pull request? Even if incomplete it's much easier to look at the diff.

[hypoactiv]

Done. This is mostly to see if I understand what you and @vladimir-ch have in mind, but may also serve as a starting point for future modifications.