slope() output advertises stale elevation units instead of degrees

[brendancol]

Describe the bug

slope() returns its result in degrees, but it copies the input DataArray's attrs verbatim via attrs=agg.attrs (xrspatial/slope.py near line 432). If the input DEM has attrs={'units': 'm'}, the slope output values are in degrees while the attrs still say units='m'. The metadata no longer matches the data.

This bites because _infer_vertical_unit_type in xrspatial/utils.py reads attrs['units'] and treats 'degrees' as an angle and 'm' as elevation. A slope output carrying units='m' gets classified as elevation, so downstream diagnostics and users are misled about what the array holds.

Expected behavior

The slope output should report the angle unit it actually produces (units='degrees'), not the input elevation unit. Other input attrs should carry through unchanged.

Steps to reproduce

import numpy as np, xarray as xr
from xrspatial import slope
agg = xr.DataArray(np.random.rand(5, 5), attrs={'units': 'm'})
out = slope(agg)
print(out.attrs['units'])  # prints 'm', should be 'degrees'

Additional context

Applies to all four backends (numpy, cupy, dask+numpy, dask+cupy) since the attrs are set in the shared public wrapper. aspect.py and curvature.py copy attrs the same way, but those are out of scope here.