added a return when null center
Merge branch 'fixedNewDesign' of https://github.com/mpi2/PhenotypeArchive.git into fixedNewDesign
Merge origin/fixedNewDesign into fixedNewDesign
Merge branch 'fixedNewDesign' of https://github.com/mpi2/PhenotypeArchive.git into fixedNewDesign
changed get experiment keys method used by graphs to use logical OR when
getting multiple pheno center or strain or metadata groups