Add B-spline and Gaussian filters

[RunDevelopment]

Hi! I needed B-spline and Gaussian for another project and implemented them myself there. Since others might also find these filters useful, I wanted to make a PR to add them to the project. I might add more in the future too, but no promises.

Changes:

• Add B-spline cubic interpolation. Coefficients come from this Wikipedia article.
• Add Gaussian filter. I basically just copied the implementation from the image crate (here and here).

Since the Type enum is not #[non_exhaustive], this is a breaking change.

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Related to #12.

[kornelski]

Thanks!