add a primitive copy method to UR::Object

It seems problematic that we don't copy id.
```
my $m = Genome::Model->get($model_id)
my $new_model = $m->copy()
```

I would expect  `$m->genome_model_id eq $new_model->genome_model_id && $m->genome_model_id eq $model_id` to be true. Otherwise we haven't really copied the object. We've just made a new one that happens to have some similarities.