Add DecompositionSeries example to ecephys tutorial

[ehennestad]

Motivation

Add example of DecompositionSeries in tutorials

How to test the behavior?

run ecephys.mlx

Checklist

• Have you ensured the PR description clearly describes the problem and solutions?
• Have you checked to ensure that there aren't other open or previously closed Pull Requests for the same change?
• If this PR fixes an issue, is the first line of the PR description fix #XX where XX is the issue number?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.16%. Comparing base (e070e20) to head (c75709d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #623   +/-   ##
=======================================
  Coverage   95.16%   95.16%           
=======================================
  Files         113      113           
  Lines        4734     4734           
=======================================
  Hits         4505     4505           
  Misses        229      229           

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