Typedspark

Roll back type annotations for `DataSet.union()` and `DataSet.intersect()`. Since the columns in a `DataSet` can be of any ordering, this means that we cannot guarantee that for example `DataSet[T].union(df: DataSet[T]) -> DataSet[T]`. For this purpose, please use `DataSet.unionByName()`.

The first official release!

Remove the hard dependency on pyspark. Instead make it a dev dependency, and give people the option to install it as an optional dependency.

By deafult, `pip install typedspark` will not install `pyspark`, since many platforms (e.g. Databricks) come with ``pyspark`` preinstalled. If you want to install `typedspark` with `pyspark`, you can run `pip install typedspark[pyspark]`