Use different cross section data for testing
Created by: paulromano
The NNDC data that we use for testing on Travis only contains a single temperature for pointwise cross sections. This means that our regression testing currently misses temperature interpolation, which has caused problems in the past. The NNDC data used to be one of the only sets of openly available ACE files, but now we have many options, and we also have pregenerated HDF5 libraries (https://openmc.mcs.anl.gov). We should switch to using one of those so that we can increase coverage in regression testing.