NXCALS-1737 python builders
#Merge request template @acc-logging-team - please have a look for approval.
New builders in pyspark world
KEY CHANGES:
- the totality of semantical validation has been pushed to java
- we can now use 'datetime' for any time indication and 'timedelta' for the duration method
- now any method that accepts varargs in java behaves identically in python: you can call fields('test') and fields(['test1', 'test2'])
- now buildDataset returns python dataset, not java dataset
- unit tests test the entire stage structure, but not types or general behaviour
- future integration tests will take care of the above
Closes NXCALS-1737