Distinguish preferred flowables and contexts in flow description.

[bl-young]

The description "Preferred flow" or "Not a preferred flow" confuses users because it does not make clear that some flows are preferred but are just using a more generic unpreferred context.

https://github.com/USEPA/fedelemflowlist/blob/d08790526b1d0c3feeaf3c8e1d98b7182a5f284c/fedelemflowlist/jsonld.py#L162-L166

When writing the flows, only the preferred state is maintained, not the distinction between preferred flowable and preferred context; these items would need to be tracked separately.