Add a note in the tutorials on PFHEstimation and FPFHEstimation about NaN values

While working on PCL, I discovered that most feature classes based on FeatureFromNormals don't check that the normals passed to the compute() function don't contain NaN values.

Radu Rusu suggested to add a note on this on tutorials.

This pull request updates the PFHEstimation and FPFHEstimation tutorials with a note and a code sample to check the normals.