BUG: Add 'center' as a data property

[wmvanvliet]

Fixes an error I get when opening a TimeViewer complaining about 'center' missing from the data properties dict.

[codecov-io]

Codecov Report

Merging #220 into master will decrease coverage by 7.34%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
- Coverage   74.15%   66.81%   -7.35%     
==========================================
  Files           8        8              
  Lines        2438     2438              
  Branches      473      473              
==========================================
- Hits         1808     1629     -179     
- Misses        456      624     +168     
- Partials      174      185      +11

Impacted Files	Coverage Δ
surfer/viz.py	62.42% <ø> (-8.76%)	:arrow_down:
surfer/tests/test_viz.py	85.24% <0%> (-11.15%)	:arrow_down:
surfer/utils.py	72.16% <0%> (-0.69%)	:arrow_down:

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[larsoner]

Thanks @wmvanvliet