Parse x[I] to use Slice(arg=x, index=i)

[walterxie]

From Alexei:

It would be nice if Slice(arg=x, index=i) could be replaced by x[I], so

Simplex r ~ WeightedDirichlet(alpha=[1.0, 1.0, 1.0], weights=[209, 210, 210]);

SiteModelGI siteModel1 = SiteModelGI(substModel=hky1, mutationRate=r[0]);
SiteModelGI siteModel2 = SiteModelGI(substModel=hky2, mutationRate=r[1]);
SiteModelGI siteModel3 = SiteModelGI(substModel=hky3, mutationRate=r[2]);