Factor out branch and bound into new package

[dpsanders]

It would be very useful to factor out branch_and_bound.jl into a new package.

But we need to watch for performance: anecdotally, I'm seeing quite a big slow down for large problems with the new tree structure?

[Kolaru]

I should be able to look into this shortly.

Do you have any example for the slow down? Hopefully there is a nice and easy fix for it.

[Kolaru]

So here it is, if you want to have a look:

https://github.com/Kolaru/BranchAndPrune.jl

Some notes:

• I still need to do the stuff to add it to the general registry
• Nothing fancy like coverall or such has been setup yet
• I have named it BranchAndPrune.jl as BranchAndBound.jl is already taken and this one does not actually do any bounding
• Several types have been renamed too

Once it's registered I'll make a PR to use it as a dependency.

[dpsanders]

Fantastic, thanks a lot!