FIX: Use average corrected PET for visualization

[mnoergaard]

This PR addresses issue #179 by improving the co-registration visualization in the html reports. It now uses an average of the motion corrected PET data instead of the petref template generated from the motion correction, which by default, is a smoothed template. The previous solution made it more difficult to assess the performance of the motion correction, whereas using the average image instead, makes it more clear to assess the performance of the co-registration.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.63%. Comparing base (133526d) to head (dcb3e19).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #183   +/-   ##
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  Coverage   80.63%   80.63%           
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  Files          84       84           
  Lines        6512     6512           
  Branches      657      657           
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  Hits         5251     5251           
  Misses       1097     1097           
  Partials      164      164           

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