added .gitattributes which marks ipynb files as documentation (not source code)
added .gitattributes which marks ipynb files as documentation (not source code)

AshleySetter/optoanalysis,AshleySetter/optoanalysis,AshleySetter/datahandling,markusrademacher/datahandling,markusrademacher/datahandling,AshleySetter/datahandling