Implement #119

[AlexVanMechelen]

This PR adds:

• Exception handling for a ZeroDivisionError, as suggested in #119

[dhondta]

@AlexVanMechelen Bad approach, I think, given the software architecture ;

• pbox.helpers.items:dict2 is the generic class that pbox.core.executable.alterations:Alterations and pbox.core.executable.features:Features rely on.
• However, only Features is concerned with your modification.
• In other implementations relying on dict2, there may be an interest to keep ZeroDivisionError caught as it may be an abnormal condition that shall be fixed instead of yielding a default value.
• Moreover, if you check pbox.core.model:Model, you will see at line 168 that -1 is the value used for filling missing values, hence the missing value shall be None.

Conclusion : I will fix this in a different way by handling the related exception in pbox.core.executable.features:Features using the None value so that it gets saved and interpreted as NaN.