Custom limits for 2D KDE

For feature parity with 1D KDE, custom limits could be supported in 2D.

I hacked an implementation by simply adding optional `xmin, xmax, ymin, ymax` arguments to `_fast_kde_2d` to override the limits inferred from the input data. Here's an example (left is current behavior of `plot_kde`, right has the same grey contours but the filled contour plot modifies `xmin` and `ymin`).


Just thought I'd throw this out there since the implementation is simple. Happy to share a reproducer (or draft a PR) if desired.

Start by tracing how plot_kde reaches _fast_kde_2d, then compare its limit handling with the 1D KDE path. Verify the requested optional xmin, xmax, ymin, and ymax behavior against the shown contour use case; done means custom limits work while existing inferred limits remain unchanged.

Written by the indexing model from the issue text.