Add an anomaly tag

[xylar]

This should make it easier to turn off analysis in situations where a reference year isn't available.

closes #844

[xylar]

Testing

I ran the analysis suite on Chrysalis. Results are here:
https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/ac.xylar/analysis_testing/chrysalis/add_anomaly_tag

(The main vs. control is missing because I messed that up, but it's nothing to do with the changes in this PR, just a user error where I ran the analysis twice in the same place.)

I also ran a version of the analysis with no_anomaly in the generate list. Here is the version that include anomalies:
https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/ac.xylar/analysis_testing/chrysalis/add_anomaly_tag/main_py3.9/ocean/
and here is the version without:
https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/ac.xylar/analysis_testing/chrysalis/add_anomaly_tag/no_anomaly/ocean/

As expected, the analysis involving anomalies is not present.