Expose pixel search to healpixdataset

[delucchi-cmu]

Feature request

Methods like pixel/order/cone search are only available on object and margin catalogs, but should be available to anything like a healpix dataset.

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• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.