Add Python Wrapper

[goodwillpunning]

Fixes [#3]. This commit adds a Python Wrapper to the DataFrame Rules Engine project.

In particular, this commit adds the following functionality in Python:

• Define a Rule using a String LOV in Python
• Define a Rule using an Expression in Python, including User-Defined Functions
• Define a Rule using a Bounds expression in Python
• Define a Rule using a MinMaxDef expression in Python

Furthermore, this commit also adds 2 new functions to the Scala RuleSet class for getting the summary and complete validation reports:

• getCompleteReport()
• getSummaryReport()

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (ef9b9fa) 87.50% compared to head (cb968bd) 86.47%.

Files	Patch %	Lines
...scala/com/databricks/labs/validation/RuleSet.scala	80.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #37 +/- ## ========================================== - Coverage 87.50% 86.47% -1.03% ========================================== Files 5 5 Lines 168 170 +2 Branches 3 2 -1 ========================================== Hits 147 147 - Misses 21 23 +2 ```

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[CLAassistant]

All committers have signed the CLA.