Description

feat: restructure existing modules, and add Column, Expression, and Functions modules

refactor existing modules to mirror the pyspark connector layout
move logic plan creation into the plan module and out of the dataframe module
create an expression module to handle Expression creation
create Column struct as a representation of a Column object
- add traits for alias and name
create functions module containing the first workings of pyspark functions
- add functions for col, pi, and coalesce
breaking change the select() trait now only excepts Vec<Column>
- tests and examples are updated to match the change

Documentation