make timestamps required for SpikeEventSeries

[bendichter]

Motivation

fix NeurodataWithoutBorders/nwb-schema#7

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 #765 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #765   +/-   ##
=======================================
  Coverage   74.63%   74.63%           
=======================================
  Files          60       60           
  Lines        6932     6932           
  Branches     1450     1450           
=======================================
  Hits         5174     5174           
  Misses       1353     1353           
  Partials      405      405

---

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[codecov]

Codecov Report

Merging #765 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #765   +/-   ##
=======================================
  Coverage   74.63%   74.63%           
=======================================
  Files          60       60           
  Lines        6944     6944           
  Branches     1455     1455           
=======================================
  Hits         5183     5183           
  Misses       1352     1352           
  Partials      409      409

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fc2a7bd...61ba149. Read the comment docs.