run with arbitrary combinations of ACIS chips

[hamogu]

marx can be run with either DetectorType=ACIS-I or DetectorType=ACIS-S. Instead, it should be possible to pick any combinations of chips, just like in a real Chandra observation (or maybe even more than 6 chips). A subtlety is that DetectorType is also used to determine the default aimpoint. If that's not clear fomr the combination of chips, some other keyword is required to set that (maybe add new keywords SIM_X, SIM_Z that default to "DEFAULT")?