ValueError: invalid literal for int() with base 10: b'Internal'

[prjemian]

apstools.devices.AD_prime_plugin2() failed with this exception. Turns out that not all cameras have a trigger mode with value "Internal" (the Dexela, for example).

In [13]: dexela_det.cam.trigger_mode.enum_strs
Out[13]: 
('Int. Free Run',
 'Int. Fixed Rate',
 'Int. Software',
 'Ext. Edge Single',
 'Ext. Edge Multi',
 'Ext. Bulb')

Priming should use a safe trigger mode. Can we assume that the first one (0) will be that safe mode? Set by number instead.