make sure aggregate centroids normalize points

[paleolimbot]

Makes sure that .Normalize() is called on points before creating the PointGeography (#119). This was already the case for non-aggregate centroids, which were created correctly.

[codecov-commenter]

Codecov Report

Merging #121 (8401275) into master (e39716e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #121   +/-   ##
=======================================
  Coverage   94.04%   94.04%           
=======================================
  Files          40       40           
  Lines        2987     2987           
=======================================
  Hits         2809     2809           
  Misses        178      178           

Impacted Files	Coverage Δ
src/s2-transformers.cpp	96.70% <100.00%> (ø)

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