Reading FAM files

[martijnvermaat]

Was: GitLab #33

• Handle twins https://git.lumc.nl/nested/nested-editor/merge_requests/48
• How to deal with all the different types of IDs? https://git.lumc.nl/nested/pedigree/issues/12
• Use all fields / data https://git.lumc.nl/nested/nested-editor/merge_requests/48

See also https://git.lumc.nl/nested/nested-editor/issues/64