make untyped dataset of references expandable

[stephprince]

Motivation

Fix #1181

To do:

• add tests

How to test the behavior?

from datetime import datetime
from uuid import uuid4
from dateutil import tz

from pynwb import NWBHDF5IO, NWBFile, H5DataIO

session_start_time = datetime(2018, 4, 25, 2, 30, 3, tzinfo=tz.gettz("US/Pacific"))

# setup NWBFile
nwbfile = NWBFile(
    session_description="Mouse exploring an open field",  # required
    identifier=str(uuid4()),  # required
    session_start_time=session_start_time,  # required
)

# add column
device = nwbfile.create_device(name="array", description="an array", manufacturer="company")

# create an electrode group
electrode_group = nwbfile.create_electrode_group(
    name="shank0",
    description="electrode group for shank 0",
    device=device,
    location="brain area",
)

# add electrode to the electrode table
nwbfile.add_electrode(group=electrode_group, location="brain area",)
nwbfile.electrodes.id.set_data_io(H5DataIO, {'maxshape': (None,), 'chunks': True})
nwbfile.electrodes.group.set_data_io(H5DataIO, {'maxshape': (None,), 'chunks': True})
nwbfile.electrodes.group_name.set_data_io(H5DataIO, {'maxshape': (None,), 'chunks': True})
nwbfile.electrodes.location.set_data_io(H5DataIO, {'maxshape': (None,), 'chunks': True})

with NWBHDF5IO("test_append_electrodes_table.nwb", "w") as io:
    io.write(nwbfile)

with NWBHDF5IO("test_append_electrodes_table.nwb", mode="a") as io:
    nwbfile = io.read()
    electrode_group = nwbfile.electrode_groups["shank0"]
    
    nchannels = 4
    for ielec in range(nchannels):
        nwbfile.add_electrode(
            group=electrode_group,
            location="brain area",
        )

    io.write(nwbfile)

with NWBHDF5IO("test_append_electrodes_table.nwb", "r") as io:
    nwbfile = io.read()
    print(nwbfile.electrodes.to_dataframe())

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?

[stephprince]

@mavaylon1 These are the changes related to our issue triaging that we will discuss testing set up for next week.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.03%. Comparing base (dedc1dd) to head (d068f8e).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1188   +/-   ##
=======================================
  Coverage   89.03%   89.03%           
=======================================
  Files          45       45           
  Lines        9883     9887    +4     
  Branches     2813     2815    +2     
=======================================
+ Hits         8799     8803    +4     
  Misses        767      767           
  Partials      317      317           

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