AffiliationSearch().affiliations field parent should be integer or None

[Michael-E-Rose]

from pybliometrics.scopus import AffiliationSearch
q = 'AF-ID(101077409)'
s = AffiliationSearch(q)
print(s.affiliations)

output is

[Affiliation(eid='10-s2.0-101077409', name='Planned Parenthood of Indiana',
                  variant='Planned Parenthood of Indiana, Inc.', documents=2, city='Indianapolis',
                  country='United States', parent='0')]

Field parent should be integer, unless it evaluates to 0, in which case it should be None. It think it is always None be now, but Scopus still returns this field.

Please change code, adapt tests and update the documentation.