Fix metastable nuclides listed for NNDC cross sections

I found today that the pre-supplied cross_sections_nndc.xml file that is used by data/get_nndc_data.py had some errors for metastable nuclides. Namely, it looks like the convention for the NNDC-supplied ACE files is that metastable nuclides have zaids that ends in 9. For example, both Am-242m and Am-244m have the designator 95249.71c. In the process of creating cross_sections_nndc.xml, these metastable nuclides were not properly recognized. In addition, Am-242 was incorrectly listed as Am-242m.

To be clear and succinct -- what was incorrect in cross_sections_nndc.xml is the *aliases*. This pull request corrects the aliases used in cross_sections_nndc.xml for metastable nuclides and Am-242. In addition, while checking the other pre-supplied cross_sections.xml files, a few errors in the cross_sections_serpent.xml file were spotted and corrected.