unyt_dask_array pickleability

[chrishavlin]

Opening this in anticipation of #185 being merged.

Currently, pickling a unyt_dask_array will strip units and return a plain dask array. Unfortunately it doesn't seem to be a trivial fix. Documenting here so that it's not forgotten...

from unyt.dask_array import unyt_from_dask
import dask
import pickle
x = unyt_from_dask(dask.array.random.random((10000,10000), chunks=100), 'm')
x2 = pickle.loads(pickle.dumps(x))
print(x)
# unyt_dask_array<random_sample, shape=(10000, 10000), dtype=float64, chunksize=(100, 100), chunktype=numpy.ndarray, units=m>
print(x2)
# dask.array<random_sample, shape=(10000, 10000), dtype=float64, chunksize=(100, 100), chunktype=numpy.ndarray>

x2 should equal x.

[neutrinoceros]

#206 may give some pointers