skip NaN in extent calculation

[rafaqz]

This PR fixes the problem where NaNs win over other coordinates to make NaN extents that don't work anywhere. Or do work in DimensionalData.jl and because NaN comparisons always return false, it just selects the maximum possible extent.

To do this I switch extrema to _nan_free_extrema, which turns out to be faster anyway.

This PR will be coupled with a PR for Extents.union and Extents.intersection to behave the same way, and for DimensionalData.jl to check for NaN extents for the rare few that still get through.

[rafaqz]

The weird thing about this (that I just realised for Extents.jl as well) is that for (X=NaN, Y=1000.0) the 1000.0 will still effect the final Y extent. Maybe this is an extreme edge case and doesn't matter much, I'm not sure.

[rafaqz]

@evetion would be good to get a review and merge on this

[evetion]

LGTM, I haven't hit this specific use case yet. Can you document the behaviour in the docstring(test?) somewhere. All NaN points will still yield a NaN extent right?

[rafaqz]

For sure, a doc comment would help. And yes all NaN still returns NaN

[asinghvi17]

LGTM, I added some documentation as well.

[asinghvi17]

We should ask what other ecosystems do at SDSL

[evetion]

I think having NaN anywhere is unwanted/unexpected, so either we can introduce a keyword to enable this behaviour, or warn when skipping NaNs. We might even error when all is NaN, as having a NaN Extent is unusable in the rest of the ecosystem?