(Very) small refractory values get lost in code generation

[mstimberg]

As mentioned on the discussion forum, very small refractory values are not handled correctly, since they are converted into code with Python's f format specifier which defaults to 6 significant digits:

>>> from brian2 import us
>>> f"{1*us:f}"
'0.000001'
>>> f"{0.6*us:f}"
'0.000001'
>>> f"{0.4*us:f}"
'0.000000'

This is of course rarely relevant (note that above the refractory period is < 1 microsecond), but still, it is unnecessary. I think I introduced this bug at some point when I modernized the code base to use format strings instead of the former % or .format syntax.

This issue is only triggered when you provide a Quantity as the refractory argument, not when you use a string directly:

group1 = NeuronGroup(..., refractory=0.4*us)    # WRONG: uses 0 as refractory period
group2 = NeuronGroup(..., refractory="0.4*us")  # WORKS