Deprecate superfluous classmethods of Centroids

[peanutfun]

Changes proposed in this PR:

• Deprecate Centroids.from_geodataframe and Centroids.from_pix_bounds
• Delete Centroids.set_raster_from_pix_bounds

Prepares for #698

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[chahank]

Good idea for simplifying the code. I would maybe just note somewhere that only regular grids will be supported (from_pix_bounds did also support grids with different x/y resolutions).

[peanutfun]

@chahank Done!