Add lerp & smoothstep to math module

[robertpfeiffer]

e.g.

import pygame

a = 0.0
b = 10.0
pygame.math.lerp(a, b, 0.5)

>>> 5.0

[Starbuck5]

I'm surprised that these functions do vectors and numbers. It seems reasonable to have the module functions just do numbers and the vector methods do vectors. There's already a way to lerp two vectors together, with vector.lerp.

[MyreMylar]

I agree with Starbuck. We should add a .smoothstep() method to the two vector classes and then have scalar/number only versions at the submodule function level.

That way there is one clear way to do it with either type of thing.

[MyreMylar]

As commented above. Please make the module level functions for scalars only, and - if you like add the vector code for smoothstep as a method for the Vector classes.

This is to keep it consistent with the already existing Vector2.lerp() & Vector3.lerp().

It has been a long ass time, for me to get around to reviewing this so also happy to take this over if you don't want to make any changes.

[MyreMylar]

Tidied this up a bit to respond to reviews.

[Matiiss]

Code looks great, seems to run fine, these are wonderful additions. Although I'm not sure why the vector methods don't use METH_FASTCALL too, but that can be changed later if necessary.