`Gedcom::Individual::relationship`: Add step-relationships

[tukusejssirs]

In Gedcom::Individual::relationship, you missed step-brother and step-sister. Difference between half-sibling and step-sibling is that half-siblings have one common parent, where step-siblings none.

I could create a PR with the following code added:

if(($self->mother() ne $other->mother()) && ($self->father() ne $other->father())) {
  return ($sex eq 'M') ? 'step-brother' : 'step-sister';
}

but when is the following code used?

return $self->relationship_down($other) || $self->relationship_up($other);

[nigelhorne]

This can only be done if step-mother and/or step-father relationships are worked out. Which currently they are not.

[tukusejssirs]

I’ll look at it out tomorrow (CEST).