Add resource reference tables

[ajtritt]

Motivation

Add the ability to track the use of external resource references across a file.

Fix #156

How to test the behavior?

import pandas as pd
from hdmf.common import ExternalResources

er = ExternalResources('terms')
er.add_ref('01234567-89ab-cdef-0123-456789abcdef', 'species', 'mouse', 'NCBI Taxonomy', '10090',
           'https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info&id=10090')
er.add_ref('fedcba98-7654-3210-fedc-ba9876543210', 'gene', 'Cacna1s', 'Mouse Genome Informatics', 'MGI:88294',
           'http://www.informatics.jax.org/marker/MGI:88294')


# Alternatively, keys can be added in bulk

df = pd.DataFrame(
    data = [['mouse', 'NCBI Taxonomy', '10090', 'https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Info&id=10090'],
            ['Cacna1s', 'Mouse Genome Informatics', 'MGI:88294', 'http://www.informatics.jax.org/marker/MGI:88294']],
    columns = ['key_name', 'resource_name', 'resource_id', 'resource_uri']
)

keys = er.add_keys(df)
er.add_ref('01234567-89ab-cdef-0123-456789abcdef', 'species', keys['mouse'])
er.add_ref('fedcba98-7654-3210-fedc-ba9876543210', 'gene', keys['Cacna1s'])

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[rly]

Looking at https://codecov.io/gh/hdmf-dev/hdmf/pull/442/diff

Can you also write tests for Table.to_dataframe(), Table.from_dataframe(), and ExternalResources.get_key()? It would be nice to get 100% diff coverage for all of this new code (we'll have to write these tests eventually and it's better done now than later), but if not, these functions should be tested at least partially.

[rly]

Otherwise, it looks good to me!