1- Added picture bee.npy to images folder.
2- Added StopIteration exception to the generate function of the GeneratorSheet and a warning to indicate the PatternGenerator do not have any patterns to push.
StopIteration in GeneratorSheet
Merge remote-tracking branch 'upstream/master'
Removed StopIteration from generate in generatorsheet.py as discussed.
Merge pull request #527 from mjabri/master

Added example NPY-format picture file.  Made GeneratorSheet catch StopIteration exceptions raised in a PatternGenerator, printing a warning rather than aborting, for PatternGenerators that may run out of patterns.