Change Contours size to be f64 instead of usize

[rmburg]

Without this, the intervals between contour lines can only be integers.
I needed a smaller interval for a figure.

To Do:

• Update changelog

[rmburg]

I added this under a new minor version in CHANGELOG.md, since it is a breaking change. Let me know if this should be changed.

[andrei-ng]

Yes please (although I find it annoying that we need to bump the major number for such small changes, we night make other ppl unhappy 😁 )

Could you fix the tests as well please 🙏.