FIX dict initialization in get_gkwh_shares_dict

[MarJene]

In Python, using:
{}.fromkeys(key_names, []) sets the SAME object list for every element in the dictionary.

Meaning:

a_dict = {}.fromkeys(['a','b'], []) 
a_dict['a'].append(0)

results in:

a_dict = {'a':[0], 'b':[0]}

As a result, when an out_refund invoice is unlinked, the sum of all gkwh rights in the invoice was marked as used for each gkwh period in the invoice.