[FR] Nodata value and incomplete ring support

[netthier]

I want to be able to generate contours that do not include lines on data/nodata borders, 'nodata' being any value either outside the grid or a specifically chosen one inside of it, commonly f32::MAX or f64::MAX, e.g. (nodata marked red):
Without nodata handling:

With nodata handling:

This also prevents a ring forming around the whole dataset, as seen in #7. While I'm aware of the workaround proposed in the issue, it is not sufficiently performant for me.
I started implementing this feature here: https://github.com/SenseLabsDE/contour-rs/tree/nodata, but the current version is relatively inefficient and results in slowdowns even if the feature is unused.

[mthh]

Yes, these are two totally valid use cases that should be supported by this library.

I think it would also be an opportunity to properly support the presence of NaN and (NEG_)INFINITY in the entry grid.