Add standardize data transform for numeric columns

[CharveeSaraiya]

Implemented a new standardize() transformation for the PETL framework that scales numeric fields to mean=0 and standard deviation=1.

• Supports multiple fields and new output columns.
• Includes full unit tests and documentation.
• Verified with pytest (all 4 tests passed) and flake8 (no style issues).

[coveralls]

Pull Request Test Coverage Report for Build 18892596153

Details

• 13 of 73 (17.81%) changed or added relevant lines in 2 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.4%) to 90.562%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
petl/transform/standardize.py	12	72	16.67%

Files with Coverage Reduction	New Missed Lines	%
petl/version.py	4	77.78%

Totals
Change from base Build 16950813691:	-0.4%
Covered Lines:	13481
Relevant Lines:	14886

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💛 - Coveralls

[juarezr]

Nice contribution.

I'll take a look as soon as I have some spare time.

As you have written typed Python code, which is a good thing, and PETL still supports older Python versions, we will need to consider transitioning to v2.0 and addressing issues like #478, #559, and #560 before.