[GSProcessing] Add default argument values for optional numerical transformation kwargs

[thvasilo]

Also adds tests and fixes some pylint errors in test file.

Issue #, if available:

Description of changes:

• Previously if a user provided a numerical transformation config where imputer, norm, or separator (for multi-num) was missing, the job would fail since the object could not be instantiated without all required arguments, although we list those args as optional.
• This PR adds default values for all optional args in the classes' init
• Also adds a couple of tests and fixes a few pylint errors in the test file.
• Also add a test for the case where the multi-numerical input is a vector value (which is the case for Parquet vector inputs)

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[jalencato]

LGTM