Check type requirements for pedigree IDs

[hannesbecher]

• Does string work, too?
• Did we document string?

[gregorgorjanc]

FYI pedigreeTools can work with numeric and character ID since these are converted to factors in its pedigree object, which gives a lot of flexibility, however I suspect that some functions in the package might not be bullet proof - see this PR I opened a while ago (which the developers have not responded to - I assume they don't have time or this is not their priority at the moment) https://github.com/Rpedigree/pedigreeTools/pull/5/files