Add validation: family multiple children with same given names

[frizbog]

Add warning validations when more than one child in a family has the same given name

[frizbog]

Enhanced this from the check in gedantic - gedantic only flags multiple children in the same family that have the same name, without checking that the children had overlapping lifespans. (It was not uncommon to re-use a name when a child died young -- the expectation of uniqueness of given names in families is a fairly recent convention). The gedcom4j version of this check includes overlapping lifespan checks.