TRestMesh node data members improved design

The TRestMesh coordinates, energy and group id data members would be better encapsulated into a single structure so that we define for example

struct MeshNode {
UInt_t GroupID;
Int_t X;
Int_t Y;
Int_t Z;
Double_t Energy;
}

Then, we would replace

    /// A vector storing the group ID of the corresponding nodes activated
    std::vector<Int_t> fNodeGroupID;
    /// A vector storing the X-dimension cell id
    std::vector<Int_t> fNodeX;
    /// A vector storing the Y-dimension cell id
    std::vector<Int_t> fNodeY;
    /// A vector storing the Z-dimension cell id
    std::vector<Int_t> fNodeZ;
    /// A vector storing the total energy inside the cell id
    std::vector<Double_t> fEnergy;

by

std::vector <MeshNode> fMeshNodes;

This issue was brought by @juanangp at PR #73

Start by locating the TRestMesh class and every use of fNodeGroupID, fNodeX, fNodeY, fNodeZ, and fEnergy; review PR #73 for the original context. Replace the separate members with the proposed MeshNode vector and update all affected usages while preserving existing mesh-node behavior.