Skip observation extraction in terminal states

[dchetelat]

This pull request avoids computing the observation function in terminal states. Following internal discussion, it also implements the "Justin trick" of temporarily removing time limits while extracting information, for making these steps atomic. (Not currently an issue but could be?) Finally, this PR makes StrongBranchingScores return a vector of size nb_vars rather than nb_cols, which was an issue detected while investigating the above issue.

[dchetelat]

@AntoinePrv I'll let you review the code, maybe you want some stylistic changes? Also, I'll run the code in parallel for the ABB project and I'll check if it solves the segfault, it should.

[gasse]

Thanks !