Adjust the default parameters of the xPowerLawInstrumentalBkg component

[lucabaldini]

Right now the default values give a background rate that is vastly smaller than the background measured in orbit---this has never been updated after launch.

Nowadays people will mainly use models of the instrumental background based on the templates derived from real observations, but arguably the simple power-law model should give by default the best approximation to the real thing, and surely something comparable with what measured on orbit.