Coherence outputs occasional pixel values above one

[scottstanie]

Noticed by me and @taliboliver , there very occasional pixels with coherence above 1

It's something with

def _take_looks_da(da: da.Array, row_looks: int, col_looks: int):
    return da.coarsen(x=col_looks, y=row_looks, boundary="trim").mean()

It's not a huge deal, but will avoid us needing to manually set the vmax when plotting coherence sometimes