reproject and merge silently drop input DataArray attrs

[brendancol]

Problem

Both reproject() and merge() in xrspatial/reproject/__init__.py build their output attrs from scratch instead of carrying the input attrs forward.

reproject() at lines ~672-697:

out_attrs = {
    'crs': tgt_wkt,
    'nodata': nd,
}

merge() at lines ~1473-1477:

attrs={
    'crs': tgt_wkt,
    'nodata': nd,
},

Anything attached to the input — units, long_name, description, scale_factor, add_offset, _FillValue, custom GeoTIFF tags, history, provenance — is dropped on the floor.

Impact

• Downstream code that reads units (e.g. unit-aware plots, validators) sees nothing.
• Packed datasets relying on scale_factor / add_offset lose the unpacking metadata. Any subsequent math is wrong.
• _FillValue / nodata overrides set by users disappear.
• Provenance text (history, description, long_name) is gone.

This is the same bug class as #1407 (sky_view_factor cellsize loss). Operations that transform a raster shouldn't strip every attr the user attached.

Fix

Merge input attrs first, then override the ones the operation legitimately changes:

out_attrs = {**raster.attrs}
out_attrs['crs'] = tgt_wkt
out_attrs['nodata'] = nd

A few input attrs become stale after reprojection and should be popped explicitly:

• transform: the affine transform changed; the old one is wrong.
• crs_wkt: we re-emit canonical crs, drop the duplicate.
• res: refers to the old grid resolution.

For merge(), use rasters[0].attrs as the base (consistent with the default strategy='first'), and fall back name to rasters[0].name before 'merged'.

Tests

xrspatial/tests/test_reproject.py doesn't currently assert any attr survives the round-trip — only 'crs' in result.attrs. Add coverage so the next regression in this area gets caught.