Modifications to get USB4000 to work:
 * Added USB endpoint definitions to oceanoptics/defines.py
 * Added USB4000 class to oceanoptics/spectrometers/USB4000.py.
   The main change is that the first 1024 pixels are sent over endpoint 6 and the rest over endpoint 2.
   This required rewriting the _request_spectrum() routine.
 * Added references to the USB4000 class to oceanoptics/utils.py