Added operator+= and operator-= to GSkymap.

The operator+= and operator-= add and subtract pixel values from one map to another by bi-linearily interpolating the values in the right hand side map.
Add log10() method that returns log10 energy in specified units.
Implement internal background model rebinning for MC sampling.

The actual background models provided with the IRF are pretty coarses, leading to measurable differences between MC simulated and modelled event distributions. To reduce the discrepancy, an internal rebinning of the background model in the spatial and energy dimensions has been implemented. Rebinning parameters are defined as maximum allowed spatial and spectral bin sizes. Parameters have been fixed to values that give satisfactory results for a specific test case (Arp220).