Uniquify

A user should be able to obtain a unique sample IDs given a set of redbiom IDs.

```bash
# get some samples within a context
$ redbiom search feature --context ${ctx} <feature>

# get unique samples within a context
$ redbiom search feature --context ${ctx} <feature> | redbiom select unique
```

The `unique` call should still yield redbiom IDs to avoid ambiguity later