add `minmax` function

[lucaferranti]

It would be nice to have a minmax function that computes both the minimum and the maximum function.

A start could be

function minmax(f::Function, X; kwargs...)
   global_min, minimiser = minimise(f, X; kwargs...)
   global_max, maximiser = maximise(f, X; kwargs...)
   return minimiser, global_min, maximiser, global_max
end

on the longer run, it could be nice to try to be smarted and solve both simultaneously and more efficiently.

@dpsanders what do you think?

[dpsanders]

I don't think there's any way to do this more efficiently than doing each separately as you suggested.