Add subset.hclust (and merge docs with plot3d.hclust)
Add complete example to hclustfc
fix docs and example for hclustfc
give groups argument a NULL default
don't set default neuronlist on attach - should be up to users to load/set
ensure that data download tests are completely independent

* they were leaving some data behind the session tempdir() which resulted
  in sequential dependence of test runs
no longer test for setting of nat.default.neuronlist