Added test for to_hierarchical_dataframe for the icephys tables

[oruebel]

This PR is in response to https://github.com/hdmf-dev/hdmf/issues/665 which identified an issue where conversion of the icephys metadata tables to a hierarchical dataframe fails if a table contains a VectorIndex column. This PR adds corresponding tests in PyNWB to make sure this case is tests here. A corresponding PR on HDMF will follow as well to fix this issue.

How to test the behavior?

From https://github.com/hdmf-dev/hdmf/issues/665

from pynwb.testing import create_icephys_testfile
from hdmf.common.hierarchicaltable import to_hierarchical_dataframe
nwbfile = create_icephys_testfile()
nwbfile.icephys_experimental_conditions.add_column('newcol', 'abc', data = [1,2,3], index = [2,3])
to_hierarchical_dataframe(nwbfile.icephys_experimental_conditions)

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