Fix float32 dtype leak in flood_depth/curve_number_runoff dask backends (#3499)

numpy and cupy cast the input to float64, but _flood_depth_dask and
_cn_runoff_dask operated on the input array without a cast, so a float32
input returned float32 on the dask and dask+cupy backends. Both functions
document a float64 output. Cast hand/p to float64 at the top of the dask
helpers so all four backends agree.

Adds float32-input dtype tests across numpy, dask, cupy, and dask+cupy.
Updates the sweep-accuracy state CSV.

Closes #3499

Author: brendancol