Resurrect the boostrap method

This MR re-enables support for the bootstrap method. To run bootstrapping, call the Sample::setNBootstrapWeights(size_t) method to tell the samples for which you need toys to internally generate vectors of toy weights. Then, in your fill methods, you can access the toy weights via double NtupleBranchMgr::getBootstrapWeight(size_t index), where index is the number of the toy weight to access. Tagging @zschilla as client for information