Add DD3to4 conversion rule for deprecated magnetics `bpol_probe` -> `b_field_pol_probe`

[maarten-ic]

See iterorganization/IMAS-Data-Dictionary#235

[maarten-ic]

To do:

Instead, we can write additional DD 3->4 conversion rules for these cases where obsolescent data nodes are used in the DD3 IDS:

1. If the source magnetics IDS contains bpol_probes and no b_field_pol_probes, convert those to b_field_pol_probes in DD4
2. If the source magnetics IDS contains method(:) and no ip(:), convert method(:)/name->ip(:)/method_name, method(:)/ip/data->ip(:)/data and method(:)/ip/time->ip(:)/time.