More testing

[bpbond]

Add a number of other tests; error checks to ISRaD.flatten; lapply_df utility function; and other minor cleanup things.

Note this currently errors because of unknown issue identified by @jb388 here, so don't merge until that's resolved.

[bpbond]

Okay it doesn't error. 🤷‍♂

[jb388]

Hm, not sure what happened, but no longer seems to error, and it's a test, so I'll go ahead and merge.