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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[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.