Change nodata from rioxarray default to nan in pyramid_reproject

[maxrjones]

Fixes #93

Rasterio (via rioxarray) uses _FillValue as the value to initialize the numpy array for the reprojected output, rather than just as the value for representing missing data when serializing to disk. As shown in #93, users expect missing data instead to be represented by NaNs when in memory. This PR fills the rioxarray nodata values with np.nan.

The _FillValue used for writing the DataTree is set later in add_metadata_and_zarr_encoding.

[codspeed-hq]

CodSpeed Performance Report

Merging #110 will not alter performance

_{Comparing reset-fill-value (7d74911) with main (948534a)}

Summary

✅ 11 untouched benchmarks

🆕 1 new benchmarks

Benchmarks breakdown

	Benchmark	main	reset-fill-value	Change
🆕	test_reprojected_pyramid_fill	N/A	14.4 s	N/A

[norlandrhagen]

Looks great to me @maxrjones! Excited to give it a try in pyramid generation / maps.