text: Title: [INF] Support Pyspark but not require it?
  Body: I'm trying to catch up on the `pyspark` conversation. But I was wondering if we were supporting `pyspark`, but not requiring it @ericmjl , @zjpoh ?
  
  I updated my local `pyjanitor` and had to download `pyspark` as the requirements changed, and it was about `215MB`. I think that's fine if it's required for development, but might not expected for users that only anticipate using `pandas`. I believe `rdkit` is currently optional for those users that will use it (maybe something else too), and I think that could be a good idea here too.
  
  Again, I haven't looked through the entire `pyspark` conversation and changes yet.