Snapshot node bug fix and docstring improvement

[neptunes5thmoon]

• remove usage of np.asscalar from snapshot node (has been removed from numpy).
• talk about option to save as zarr file in docstring

Pre-Merge Checklist:

• [ ] if this PR adds a feature: request merge into the latest development branch (vX.Y-dev)
• [ ] if this PR fixes a bug: request merge into the latest patch branch (patch-X.Y.Z)
• [ ] PR branch name is short and describes the feature/bug addressed in this PR
• [ ] commit messages are consistent
• [ ] changes reviewed by another contributor
• [ ] tests cover changes
• [ ] all tests pass

[pattonw]

Looks pretty good to me. I'll create a patch branch and merge it there

[pattonw]

merged in branch patch-1.3.4