Allow the static background in EBSD metadata to be a dask array in EBSD.remove_static_background()

[hakonanes]

When the static background is a dask array, I get this error when calling s.remove_static_background():

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/tmp/ipykernel_131261/2987853041.py in <module>
----> 1 s.remove_static_background()

~/kode/kikuchipy/kikuchipy/signals/ebsd.py in remove_static_background(self, operation, relative, static_bg, scale_bg)
    506                 md = self.metadata
    507                 ebsd_node = metadata_nodes("ebsd")
--> 508                 static_bg = da.from_array(
    509                     md.get_item(ebsd_node + ".static_background"), chunks="auto"
    510                 )

~/miniconda3/envs/kp-dev/lib/python3.8/site-packages/dask/array/core.py in from_array(x, chunks, name, lock, asarray, fancy, getitem, meta, inline_array)
   3097     """
   3098     if isinstance(x, Array):
-> 3099         raise ValueError(
   3100             "Array is already a dask array. Use 'asarray' or " "'rechunk' instead."
   3101         )

ValueError: Array is already a dask array. Use 'asarray' or 'rechunk' instead.