Add polynomial s shape impact function

[chahank]

Changes proposed in this PR:

• Adds a generic method to make polynomial s-shape impact functions ImpactFunc.from_poly_s_shape() as proposed by Emanuel et al. (2011). This methods generalizes the ImpfTropCyclone.from_emanuel() method.

This PR fixes #

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• Descriptive pull request title added
• Source branch up-to-date with target branch
• Documentation updated
• Tests updated
• Tests passing
• No new linter issues
• Changelog updated

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[peanutfun]

Thanks for the addition! I think the function will be very useful.

Please improve the documentation and the tests. One linter issue can also easily be fixed.

[peanutfun]

Check actual error

Suggested change

	with self.assertRaises(ValueError):
	with self.assertRaisesRegex(ValueError, "Exponent value"):

[chahank]

Thanks for the correction! Why should we priorize using assertRaisesRegex ?

[peanutfun]

Because it tests the actual error message. assertRaises would succeed with any ValueError thrown, but you want to test for a specific one here.

Note that this is basically a shortcut for the idiom we were using so far:

with self.assertRaises(Exception) as cm:
    do_stuff()
self.assertIn("Text", str(cm.exception))

[chahank]

Makes total sense, thanks!

[chahank]

Ok, I fixed all that you proposed, thanks! I also fixed one test and made sure that for threshold = half_point the method returns mdd=0 (else there was a divide by 0).

[peanutfun]

Great work! Will merge.