Reduce memory footprint

[ktnr]

Memory usage increases quite rapidly and can reach up to several hundreds of GB for hard instances. The complete search tree is kept in memory during the execution of the program. However, explored parts of the tree could be released to free up memory.

Doing so would require reorganizing node selection, which is currently managed via IDs, e.g. in backtracking:

TwoStepBranchingProcedure/tsbp/src/BranchAndBound.cpp

Line 855 in 2fa7152

Node& nodeToDeactivate = this->tree[nodeIdToDeactivate];

Implementing #4 would also help in this regard.