Divide by zero bug in AbstractConvectionTerm

[opensourcefan314]

AbstractConvectionTerm tries hard to make sure the peclet number is never exactly 0 by replacing hard 0s with +/- 1e20.

However this line can fail to eliminate all zeros:
https://github.com/usnistgov/fipy/blob/master/fipy/terms/abstractConvectionTerm.py#L139
diffCoeff = diffCoeff - (diffCoeff == 0) * geomCoeff / pecletLarge
The problem occurs when an entry of geomCoeff is 0, or is small enough that geomCoeff / pecletLarge underflows.
I'm not sure what the best principled fix would be, but a hacky fix is to follow this line with an additional line
diffCoeff = diffCoeff - (diffCoeff == 0) / pecletLarge

Here is a gist that triggers the divide by 0:
https://gist.github.com/opensourcefan314/c7fa68bf44e069406f617439a9417518

Executing the gist prints this warning:
/fipy/variables/variable.py:1143: RuntimeWarning: invalid value encountered in true_divide
return self._BinaryOperatorVariable(lambda a, b: a / b, other)

In this case, the divide by zero seems harmless, in the sense that it doesn't propagate nans or infs to the solver output.