renamed fields for repurposing (issue 2)
resolved some conflicts with schema change
removed unnecessary uniqueness constraint
added schema migration
restored non-null attribute where it belongs
fixed migration order
interim fix for genotype_value
added convenience legacy accessors
new schema now works with loaders, tests; support functions extended to accommodate wide VCFs
fixed formatting
Merge branch 'master' into issue-2
Merge branch 'master' into issue-2
made changes specified in PR
fixed flake8 violation
applied migration changes from 1f67a2ab4b6915f6b88821934d740e765161467a
Add signal to support additional foreign keys on Sample

Signed-off-by: Adam Wenocur <wenocur@email.chop.edu>
Merge pull request #14 from awenocur/add-signal

Add signal to support additional foreign keys on Sample
Merge branch 'master' into issue-2
index on issue-2: 69dd564 Merge branch 'master' into issue-2
WIP on issue-2: 69dd564 Merge branch 'master' into issue-2