Generalize expand_pad_width to expand_pad_value (#4150)
Generalize expand_pad_width to expand_pad_value (#4150)

As we use this function both with widths and statistical values, which
are not widths, rename it to something more fitting like
`expand_pad_value`.
mrocklin/dask,blaze/dask,jcrist/dask,jcrist/dask,jakirkham/dask,dask/dask,mrocklin/dask,jakirkham/dask,ContinuumIO/dask,dask/dask,ContinuumIO/dask,blaze/dask