D5a_module does not support D5a_module.range_8V_uni

[drcrielaard]

In the get_stepsize() function of the D5a_module

The snippet

if self.span[DAC] == (D5a_module.range_4V_bi or D5a_module.range_8V_uni):
    return 8.0/(2**18)

Does not get run in the case self.span[DAC] == D5a_module.range_8V_uni, since (D5a_module.range_4V_bi or D5a_module.range_8V_uni) == 1.

This should be

if self.span[DAC] == D5a_module.range_4V_bi or self.span[DAC] == D5a_module.range_8V_uni:
    return 8.0/(2**18)

Since the 8V settings rarely get used, it seems like this never popped up.