Add header guard to datatypes.h

[fmauch]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3e25ffc) 72.05% compared to head (a13e3a3) 72.09%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
+ Coverage   72.05%   72.09%   +0.03%     
==========================================
  Files          71       71              
  Lines        2652     2652              
  Branches      337      337              
==========================================
+ Hits         1911     1912       +1     
  Misses        554      554              
+ Partials      187      186       -1     

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[RobertWilbrandt]

Good change that prevents simple user errors. I just realized that we are not 100% consistent with pragma once vs. header guard in our header files, maybe something to keep in mind in the future.