Rewrite `__len__` in terms of `__iter__`
Rewrite `__len__` in terms of `__iter__`

Reuse the `__iter__` method in determining the length of the
`DistributedArrayStore` instance. Namely count each entry as `1` and sum
them to get the total length.

DudLab/nanshe_workflow,nanshe-org/nanshe_workflow