Command-line / API features to use weight subsets or ignore weights entirely
The --skip-weights feature is handy, but doesn't do what it sounds like: rather it uses the 0th weight and ignores the rest.
We could do with making that more precise and more flexible, i.e. the user gets to say which weight (or subset) to use. And either an empty subset or a separate (overriding) mechanism to say "treat all weights as unity". Subsets would be nice, most obviously as a comma-separated command-line (and RIVET_WEIGHT_INDEX env variable) string, mapped into a vector in the API, but at least being able to specify a single weight index (either positional or named) would be a big step and not too difficult to do. The RIVET_WEIGHT_INDEX should be reintroduced for continuity, since we still want the functionality it provided access to.