Consider relaxing the Sampler.parameters API

**Application**
At the moment, the `Sampler` ABC asks that the `Sampler.parameters` attribute be a dict of the form `{kwarg: [relevant_property, ...], ...}`. In practice though we rarely use the relevant properties list and almost all samplers look like `{'beta_range': [], 'num_reads': []}`

**Proposed Solution**
I propose that `Sampler.parameters` should return a [Collection](https://docs.python.org/3/library/collections.abc.html#collections.abc.Collection) of accepted sampler arguments. This supports our current form but the above example could instead be `{'beta_range', 'num_reads'}`.