change GFF3 type for splice sites

5' splice sites now have the type 'five_prime_cis_splice_site' (instead of
'five_prime_splice_site') and 3' splice sites the type
'three_prime_cis_splice_site' (instead of 'three_prime_splice_cite') to comply
with the new version of SOFA (Sequence Ontology Feature Annotation).