Add metadata to the library

Added a little bit of metadata information to the library. This is stored as two objects of type std::map<std::string, int> and std::map<std::string, double>. The first one has three keys, energyBins, thetaBins and phiBins, each giving number of bins in the given variable in the library. The second one has keys minE, maxE, minTheta and maxTheta giving the range in energy and angle for which library is constructed.

cc @mrama