[MRG] add transformer for substraction of datetime variables

[solegalli]

closes #486
closes #359
closes #330

• add few more tests, ie non fitted error
• finish user guide

[codecov]

Codecov Report

Merging #632 (e7ccd4c) into main (9db7124) will decrease coverage by 0.08%.
The diff coverage is 93.93%.

❗ Current head e7ccd4c differs from pull request most recent head 75cac5a. Consider uploading reports for the commit 75cac5a to get more accurate results

@@            Coverage Diff             @@
##             main     #632      +/-   ##
==========================================
- Coverage   97.90%   97.83%   -0.08%     
==========================================
  Files          98       99       +1     
  Lines        3588     3653      +65     
  Branches      695      706      +11     
==========================================
+ Hits         3513     3574      +61     
- Misses         28       30       +2     
- Partials       47       49       +2     

Impacted Files	Coverage Δ
feature_engine/datetime/datetime_subtraction.py	93.75% <93.75%> (ø)
feature_engine/datetime/__init__.py	100.00% <100.00%> (ø)

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[solegalli]

Hey @glevv

If you have time, would you be able to have a quick look at this new class? maybe just class and tests and ignore the doc files?

that would be amazing :)