The defoult choice between open vs periodic boundary condition should not depend on the `calculate_alpha` keyword

[nscolonna]

The default choice between open vs periodic boundary condition (BC) - i.e. if we want to consider or not image charge corrections -should only depend on whether we are doing a 0D calculation or not. At the current stage the default value of the BC depends on the calculate_alpha workflow keyword:
https://github.com/epfl-theos/koopmans/blob/1b656f961337bb9cbd696a0207e286f63f6e0d73/src/koopmans/workflows/_workflow.py#L438C9-L438C48

Suggested solution: Remove the if condition related to the calculation of alpha (calculate_alpha) and keep only the one related to the dimensionality (periodic)