Add tolerance support for align/combine/concat operations

[dcherian]

Closes #16

Summary

• Add set_options/get_options context manager for configuring transform tolerance
• Add BoundingBox.isclose() method for tolerance-aware comparison
• Default transform_rtol=1e-12 handles typical floating-point noise in geotransforms

Usage

import rasterix

with rasterix.set_options(transform_rtol=1e-9):
    result = xr.concat([ds1, ds2], dim='x')

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