vpImage::getStdev specialization

[rolalaro]

• Template specialization for vpRGBa and vpRGBf of the vpImage::getStdev method
• Added a test file for vpImage::getSum, vpImage::getMeanValue and vpImage::getStdev methods

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.81%. Comparing base (7328caa) to head (15f6f30).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1349      +/-   ##
==========================================
+ Coverage   52.66%   52.81%   +0.15%     
==========================================
  Files         410      410              
  Lines       52646    52704      +58     
==========================================
+ Hits        27724    27834     +110     
+ Misses      24922    24870      -52     

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