b+树learning index

##AC++11implementationoftheB-Treepartof"TheCaseforLearnedIndexStructures"Aresearch**proofofconcept**thatimplementstheB-Treesectionof[TheCaseforLearnedIndexStructures](https://arxiv.org/pdf/1712.01208.pdf)paperinC++.Thegeneraldesignistohaveasinglelookupstructurethatyoucanpar