implement SNR estimation to choose between PCA or Projective projection

BUG: input image pixel type must be set to double in RAndNIRVegetationIndexImageFilter to handle NIR correct values, which can be greater than 255.

ENH: Adding a filter to perform elevation map estimation from a pair of stereo image in sensor geometry

COMP: remove use of otb::StringStream since there is already an ITK equivalent, and it will disappear in ITKv4