add option to use a custom html field generation method

[stephprince]

Motivation

Fix #1010 with accompanying changes in pynwb (see related PR NeurodataWithoutBorders/pynwb#1831).

These changes will check whether a value has a _generate_field_html method and use that method if it is an option when generating html representations.

How to test the behavior?

See below for an example from the related issue in which timestamps are linked across multiple spatial series.

from pynwb import NWBHDF5IO
from pynwb.testing.mock.file import mock_NWBFile
from pynwb.testing.mock.behavior import mock_SpatialSeries, mock_Position
import numpy as np

nwbfile = mock_NWBFile()

test_timestamps = np.zeros((1000,))
test_data = np.random.rand(*(1000, 3))
spatial_series1 = mock_SpatialSeries(name="test1", rate=None, data=test_data, timestamps=test_timestamps)
spatial_series2 = mock_SpatialSeries(name="test2", rate=None, data=test_data, timestamps=spatial_series1)
spatial_series3 = mock_SpatialSeries(name="test3", rate=None, data=test_data, timestamps=spatial_series1)
spatial_series4 = mock_SpatialSeries(name="test4", rate=None, data=test_data, timestamps=spatial_series1)
position = mock_Position(spatial_series=[spatial_series1, spatial_series2, spatial_series3, spatial_series4])

nwbfile.create_processing_module("behavior", description="contains processed behavior data")
nwbfile.processing["behavior"].add(position)

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

read_io = NWBHDF5IO("test.nwb", "r")
nwbfile_in = read_io.read()

nwbfile_in

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 88.53%. Comparing base (8ad4db2) to head (66ea6bb).
Report is 59 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1038   +/-   ##
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  Coverage   88.53%   88.53%           
=======================================
  Files          45       45           
  Lines        9603     9605    +2     
  Branches     2731     2732    +1     
=======================================
+ Hits         8502     8504    +2     
  Misses        778      778           
  Partials      323      323           

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