Update Th-232 series notebook example

[markbandstra]

The NNDC data querying example notebook needed two updates:

• It was using a deprecated pandas method (DataFrame.append), replaced with pandas.concat
• The radiation from the thorium series started at Th-228 for reasons obscure to me (maybe because we had a Th-228 source in the lab), so if you refer to it for the natural thorium background lines it is missing some important ones.

[markbandstra]

@jvavrek seems like the only problem is a liccheck issue and I'm not sure why it's even checking the pillow version since it's not a dependency.

Run Python License Checker...............................................Failed
- hook id: liccheck
- exit code: 255

gathering licenses...
45 packages and dependencies.
check authorized packages...
44 packages.
check unknown packages...
1 package.
    pillow (11.0.0): ['CMU License (MIT-CMU)']
      dependency:
          pillow << matplotlib

[jvavrek]

CMU is fine, so you could add exactly CMU License (MIT-CMU) to the .liccheck.ini

We could also disable checking the downstream dependencies in the tool.