Pull request: plot_view, some optimization, NumpyFile default file and GeneratorSheet.generate activity assignment

- Added plot_view to sheet. Flag that controls if sheet activity to be display in plot|activities. This allow turning off so
me of the activities to be displayed in case of a number of larger number sheets which makes scolling very unplesant.
- Added test on linear_slope in HomeostaticResponse._apply_threshold so if it is 1.0 so no-op.
- Fixed what seems to be a bug (please check bug or feature) in GeneratorSheet.generate in the assignment of self.activity r
egardless of the return value of self.input_generator() (which may be none if input_generator does not have a pattern which 
was assiging Nan to the activities.
- added numpy file numpy_array_feret_photo.npy in images folder as it seems this is where the FileImage test files are. This
 is not pretty as the imagen package is refering to a file outside the package! Should consider  changing both to refer to a
 local folder of imagen.