Fix [-Wrange-loop-construct] compilation warnings from clang10
Fixes warnings like
warning: loop variable 'P' of type 'const Rich::HPDImage::Pixel' creates a copy from type 'const Rich::HPDImage::Pixel' [-Wrange-loop-construct]
for ( const auto P : boundary ) { fitter.addPoint( (double)P.col, (double)P.row ); }
^
@jonrob FYI