Fix math domain error in decaying poll rate

[teojgo]

• Check that self._a is not smaller that a minimum allowed value.

Fixes #77

[victorusu]

Does it make sense to have this variable defined in the settings.py?
Or Have it defined capitalized at the beginning of the file?

[vkarak]

@victorusu Generally, yes, but not in this issue. I will create another one.

[teojgo]

I added this one so that I do not enter a magic number in the middle of the code.

[vkarak]

@victorusu Another problem with exposing _min_a as-is is that it doesn't have a physical meaning. It is not the minimum poll rate.

[vkarak]

I disagree with this solution. _min_a has no meaning and it is arbitrary. The problem here is that the initial rate is less than the minimum rate. Ideally, we would like to change the function and make it an increasing logarithmic. Since this is a bit too much, I would suggest the following:

if abs(self._min_rate - init_rate) < eps:
    self._a = 0
    self._c = 0
else:
    # as of now

The above function will make the subsequent calls to __call__() return always the minimum rate. The eps should have such a value to guarantee that math.log() doesn't throw.

[vkarak]

@jenkins-cscs retry daint