Skip genotype models without references.

The reference can't match our reference if it doesn't exist!
Create the bridge entities more explicitly.

UR tries to find an "add_analysis_project" subroutine, which does
not exist.  So call something that does exist instead.
Merge pull request #330 from tmooney/explicitly_create_bridge

Create the bridge entities more explicitly.
Merge pull request #327 from tmooney/skip_genotypes_without_references

Skip genotype models without references.