Enh/add links to elec grp

[bendichter]

Motivation

Adds links to ElectrodeGroup that facilitates queries between ElectrodeGroups and recorded data
depends on #827

How to test the behavior?

ElectrodeGroup('elec1', 'a test ElectrodeGroup', 'a nonexistent place', Device(),
               event_waveform=EventWaveform(), lfp=LFP())

Checklist

• Have you checked our Contributing document?
• Have you ensured the PR description clearly describes 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 #XXX notation where XXX is the issue number ?

[codecov]

Codecov Report

Merging #828 into dev will increase coverage by <.01%.
The diff coverage is 68.42%.

@@            Coverage Diff             @@
##              dev     #828      +/-   ##
==========================================
+ Coverage   73.77%   73.78%   +<.01%     
==========================================
  Files          60       60              
  Lines        7184     7193       +9     
  Branches     1514     1517       +3     
==========================================
+ Hits         5300     5307       +7     
- Misses       1457     1458       +1     
- Partials      427      428       +1

Impacted Files	Coverage Δ
src/pynwb/ecephys.py	97.41% <100%> (+0.06%)	⬆️
src/pynwb/form/build/map.py	72.29% <60%> (+0.1%)	⬆️
src/pynwb/form/spec/spec.py	78.41% <60%> (-0.23%)	⬇️

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[bendichter]

Oliver wants to talk about this PR on Thursday before we merge

fixed

[ajtritt]

@bendichter @oruebel this has gone the way of #822 and #825. We should decide what to do on these