Added a check for the name of the ACE data to be read.

Hi!
I got erroneous results (keff off by 300 pcm or so) when using OpenMC with my own cross section libraries. It turned out that the xs listing file for OpenMC (cross_sections.xml) was broken and, hence, OpenMC read the wrong S(a,b) data from the ACE file containing numerous entries for different moderators.

I generated the broken xs listing file using convert_xsdata.py, which does not seem to work properly in the case of S(a,b) data. I don't speak any python, so I could not fix the script, but instead I added a check to ace.F90 that ensures that the ACE data to be read in corresponds to the data requested in materials.xml. With the modification, this kind of ACE-reading errors should be avoided in the future.
Tuomas