Add diagnostic information to Solution objects

[gabrielgellner]

the solvers generate useful information like the number of function evaluations etc. This information needs to be added to the output objects. What needs to be considered is if it is better to have a type field for this kind of information, or should each piece of information be top level?

Look at examples from Optim.jl and NLopt.jl.

[gabrielgellner]

Looking at how matlab deals with this when the sol = ode45(...) version is used then the returened struct has the field stats which contain the fields nfevals nsteps nfailed currently we collect nsteps (the number of accepted steps) and nfailed (the number of failed steps). We also collect information on dts and errs which I think is related to the tolerances. I need to add the nfevals. As well as checking what common naming conventions for these fields are in Julia.