Add getters for excitation and ionisation rates and levels.

Example:

#include <iostream>

#include <TCanvas.h>
#include <TGraph.h>
#include <TROOT.h>
#include <TApplication.h>

#include "Garfield/MediumMagboltz.hh"
#include "Garfield/FundamentalConstants.hh"

using namespace Garfield;

int main(int argc, char * argv[]) {

  TApplication app("app", &argc, argv);
 
  // Load a gas file which includes excitation and ionisation rates.
  MediumMagboltz gas;
  gas.LoadGasFile("ar_93_co2_7.gas");
  // gas.PrintGas();

  auto nIon = gas.GetNumberOfIonisationLevels();
  auto nExc = gas.GetNumberOfExcitationLevels();
  std::cout << nIon << " ionisation levels:\n";
  for (size_t i = 0; i < nIon; ++i) {
    std::string label;
    double energy = 0.;
    gas.GetIonisationLevel(i, label, energy);
    std::cout << label << ", " << energy << " eV.\n";
  }
  std::cout << nExc << " excitations:\n";
  for (size_t i = 0; i < nExc; ++i) {
    std::string label;
    double energy = 0.;
    gas.GetExcitationLevel(i, label, energy);
    std::cout << label << ", " << energy << " eV.\n";
  }
  std::vector<double> efields;
  std::vector<double> bfields;
  std::vector<double> angles;
  gas.GetFieldGrid(efields, bfields, angles);
  const auto nE = efields.size();
  TGraph gIon(nE);
  TGraph gExc(nE);
  for (size_t i = 0; i < nE; ++i) {
    double fsum = 0.;
    for (size_t j = 0; j < nIon; ++j) {
      double f = 0.;
      gas.GetElectronIonisationRate(j, i, 0, 0, f);
      fsum += f;
    }
    gIon.SetPoint(i, efields[i], fsum);
    fsum = 0.;
    for (size_t j = 0; j < nExc; ++j) {
      double f = 0.;
      gas.GetElectronExcitationRate(j, i, 0, 0, f);
      fsum += f;
    }
    gExc.SetPoint(i, efields[i], fsum);
  }
 
  gIon.SetMarkerStyle(20);
  gIon.SetMarkerColor(kRed + 1);
  gExc.SetMarkerStyle(21);
  gExc.SetMarkerColor(kBlue + 1);
  TCanvas c1("c1", "", 600, 600);
  c1.cd();
  gExc.Draw("apl"); 
  gIon.Draw("plsame");
  app.Run(true);
}