Make SpatialSeries reference_frame optional

[rly]

Motivation

Fix #1985

How to test the behavior?

ss = SpatialSeries(name="a", data=[1, 2, 3], timestamps=[1, 2, 3])

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?
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.87%. Comparing base (e938202) to head (703756b).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1986   +/-   ##
=======================================
  Coverage   91.87%   91.87%           
=======================================
  Files          27       27           
  Lines        2695     2695           
  Branches      703      703           
=======================================
  Hits         2476     2476           
  Misses        145      145           
  Partials       74       74           

Flag	Coverage Δ
integration	72.69% <ø> (ø)
unit	83.33% <ø> (ø)

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