fix rare bug in identify subdomain

If a subdomain, whose elements touch multiple bulk elements, already has bulk_element_ids in cell_data (which should usually never happen), the new bulk_element_ids don't get written as intended in the field_data, but in a cropped and faulty version in cell_data.

With the small change identifySubdomains will corretly throw an error:

"[error] A property of the name 'bulk_element_ids' is already assigned to the mesh."