Optics order checking when creating a `diagnostic`: better warning msg + minor bugfix for `collimator` cameras

[dvezinet]

Main changes:

• add_diagnostic():

  • was testing all centers of all pixels for optics order, even when called for a single pixel in the case of collimator cameras
  • now print a more informative warning msg

• no numerical error found

Issues:

Fixes, in devel, issue #956

Examples:

It should be noted that the etendue * length overestimates the VOS by a factor 10 because the sensors is 3 times larger than the collimator and analytical etendue formula assumes that the whole pixel is illuminated, which is clearly not the case here, by a factor 9 approximately