Use keywords in df.to_hdf call

[abostroem]

What is the problem?

Pandas now issues a warning that only the filename will be allowed without a keyword in Pandas 3.0. We need to specify the key with key= everywhere we write an hdf5 file

Location of problem (optional)

https://datacarpentry.github.io/astronomy-python/04-motion.html, https://datacarpentry.github.io/astronomy-python/03-transform.html