Replace zeroes in StorageCapacity_MWh with NaN

[irm-codebase]

My version (0.5.11) fills StorageCapacity_MWh with 0.0 instead of NaN.

This does not really make a lot of sense for storage facilities. I propose to replace these zeroes with NaN to avoid confusion, and to make users fill those values in if necessary through their own methods.

Here is an example of facilities in Italy:

>>plants.loc[(plants["Set"]=="Store")]["StorageCapacity_MWh"]
id
11         0.0
41         0.0
42         0.0
54         0.0
67         0.0
         ...  
5514       0.0
5569       0.0
5617       0.0
5644       0.0
5665    2600.0
Name: StorageCapacity_MWh, Length: 99, dtype: float64