Write scalar datasets with compound data type

[stephprince]

Motivation

This came up in NeurodataWithoutBorders/pynwb#1770, where ElectrodeGroup.position is a scalar dataset with a compound data type. There was an issue with write, so the written dataset was not scalar and/or values would be duplicated.

Once this PR is incorporated, the related pynwb PR needs additional changes to the ElectrodeGroup position validation so that the position input is converted to a scalar with a compound data type. For example, something like:

if args_to_set['position']:
        if len(np.shape(args_to_set['position'])) > 1 or \
        (len(np.shape(args_to_set['position'])) == 1 and len(args_to_set['position']) != 3):
            raise ValueError(f"ElectrodeGroup position argument must have three elements: x, y, z,"
                            f"but received: {args_to_set['position']}")
        else:
            args_to_set['position'] = np.array(args_to_set['position'], dtype=[('x', float), ('y', float), ('z', float)])

How to test the behavior?

from pynwb import NWBFile, NWBHDF5IO
from uuid import uuid4
from datetime import datetime

nwbfile = NWBFile(
    session_description="no description.",
    session_start_time=datetime.now(),
    identifier=str(uuid4()),
)

device = nwbfile.create_device(name="Device", description="no description.")

electrode_group = nwbfile.create_electrode_group(
    name="ElectrodeGroup",
    description="no description.",
    device=device,
    location="unknown",
    position=[(1., 2., 3.)],  # --> dataset shape = (1,)
    # position=(1, 2, 3)      # --> dataset shape = (3,)
)
)

with NWBHDF5IO('test.nwb', 'w') as io:
    io.write(nwbfile)
    
with NWBHDF5IO('test.nwb', 'r') as io:
    read_nwbfile = io.read()
    
    print(read_nwbfile.electrode_groups['ElectrodeGroup'].position.shape)  # (1,)

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.89%. Comparing base (2b167ae) to head (2169fdb).
Report is 26 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1176   +/-   ##
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  Coverage   88.89%   88.89%           
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  Files          45       45           
  Lines        9839     9844    +5     
  Branches     2797     2799    +2     
=======================================
+ Hits         8746     8751    +5     
  Misses        776      776           
  Partials      317      317           

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[rly]

Looks good to me!