Implement GUB branching

[AntoinePrv]

Pull request checklist

• I have open an issue to discuss the proposed changes. Fix Multi variable branching #158
• I have modified/added tests to cover the new changes/features.
• I have modified/added the documentation to cover the new changes/features.
• I have ran the tests, checks, and code formatters.

General idea

For integer variables x_i provided by the user, and their LP relaxation value x_lp_i,

• If s := sum(x_lp_i) is fractional, then branch on sum(x_i) <= floor(s) & sum(x_i) >= ceil(s)
• If s := sum(x_lp_i) is integer, then branch on sum(x_i) <= s - 1 & sum(x_i) == s & sum(x_i) >= s+ 1

Proposed implementation

• When n_vars == 0, fail;
• Otherwise (including when n_vars == 1):
  • Fail if any variable is continuous or fixed
  • if sum(x_lp_i) is infinite, what then?
  • if sum(x_lp_i) is integer, branch on three nodes
  • else branch on two nodes

[AntoinePrv]

When given all the action set

_, action_set, _, done, _ = env.reset(next(instances))
while not done:
    _, action_set, _, done, _ = env.step(action_set)

And that their sum is integral, three nodes are created. But since no variable get fixed, they all appear again in the action_set and the code loops indefinitely.

[gasse]

Minor comments about variable names and a few typos. High quality PR !

[gasse]

Why is this argument called ones ? Does it have to be a vector of ones ? If so, why is it an argument ?

[AntoinePrv]

Ha, yes it's literally a vector of ones. It's being reused between the left and right child...