Use better datastructure to store CellMapping

[tristan0x]

This iteration over CellMappings below should be avoided with a better datastructure by using an unordered_map or a std::vector sorted by gid.

https://github.com/BlueBrain/CoreNeuron/blob/557f2566427908b68918403da87e3af2af65914c/coreneuron/utils/reports/nrnsection_mapping.h#L157-L164