A couple of patches to relatively obscure xspec bugs

12.7.0r	When using C-stat, if spectra with no backgrounds and different exposures and/or areascales are collected into the same data group, the renorming operation will miscalculate and can disrupt the fit. Our thanks to Jeremy Sanders for pointing this out. Report added on Dec 06, 2011.
12.7.0s	Fix for bug which caused errors in the apec model when running v1.x of AtomDB and the wilm abundance option. Report added on Dec 08, 2011.