Make PySpark an optional dependency

[MrPowers]

PySpark is a heavy dependency and shouldn't be required for users that just want to create Pandas DataFrames.

pip install farsante should not install PySpark.

pip install farsante[pyspark] should be the PySpark installation.

Here is how to install optional dependencies with Poetry. I am guessing this will work.

[MrPowers]

I think Poetry dependency groups should work well for this task.