Add documentation for ClassTag in saveAvro method

[hannahbarfield]

The saveAvro method in the file GenomicDataset.scala currently lacks documentation on the use of ClassTag, which is important for handling specific Avro record types. Adding this documentation will improve code clarity and usability.

[heuermh]

Hello @hannahbarfield!

Thank you for pointing this out -- there are probably a lot of methods that could use additional documentation. Pull requests of course are always welcome 😁