BoutDataArray.interpolate_parallel() result is missing regions

[johnomotani]

When using the BoutDataArray version of interpolate_parallel(), the result returned does not have the regions attribute created. regions is only created in the BoutDataset version. This means that the result does not support various useful methods, e.g. plotting on the poloidal plane.

Not sure this is easily fixable: I suspect there were reasons a full Dataset was needed to create the regions, although it may just have been convenience and to avoid duplicated code.

Possibly the best solution would be to fix #164, which should take care of this issue as a by-product.

The current workaround is to just use the BoutDataset version, and then get the DataArray out of the result. I.e. replace

n = ds["n"]
n_highres = n.bout.interpolate_parallel()

with

ds_highres = ds.bout.interpolate_parallel("n")
n_highres = ds_highres["n"]