Feat cxx 11 min standard

[fspindle]

• Bump cxx11 as minimum standard
• Use nullptr instead of NULL
• Fix build around cv::canny usage when OpenCV < 3.2.0
• Fix build of vpCircleHoughTransform when nlohmann json not available

[codecov]

Codecov Report

Attention: 193 lines in your changes are missing coverage. Please review.

Comparison is base (549fb2c) 54.38% compared to head (daaf054) 54.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1267      +/-   ##
==========================================
- Coverage   54.38%   54.38%   -0.01%     
==========================================
  Files         400      400              
  Lines       50484    50486       +2     
==========================================
  Hits        27456    27456              
- Misses      23028    23030       +2     

Files	Coverage Δ
...dules/core/include/visp3/core/vpCameraParameters.h	100.00% <ø> (ø)
modules/core/include/visp3/core/vpColVector.h	79.16% <100.00%> (ø)
modules/core/include/visp3/core/vpDebug.h	0.00% <ø> (ø)
modules/core/include/visp3/core/vpException.h	100.00% <ø> (ø)
...ules/core/include/visp3/core/vpHomogeneousMatrix.h	100.00% <ø> (ø)
modules/core/include/visp3/core/vpImage.h	82.53% <100.00%> (ø)
modules/core/include/visp3/core/vpImageConvert.h	93.33% <ø> (ø)
modules/core/include/visp3/core/vpImagePoint.h	100.00% <100.00%> (ø)
modules/core/include/visp3/core/vpImageTools.h	87.66% <100.00%> (ø)
modules/core/include/visp3/core/vpLinProg.h	100.00% <ø> (ø)
... and 78 more

... and 22 files with indirect coverage changes

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