instrument restructuring

• instrument module now has much more rich functionality (SubarrayDescription, TelescopeDescription, OpticsDescription classes added
• no more need to construct CameraGeometry and others manually, they are created in the hessio_event_source
• all new code should use event.inst.subarray
• the old inst.tel_pos, inst.optics_foclen, etc, will be phased out in the next point release (but still exist in this release)