Sampling of alternatives: additional features

[smmaurer]

Feature wish list for sampling of alternatives in the MergedChoiceTable utility:

1. Availability of alternatives
2. Accepting callable for on-the-fly calculation of weights, availability, and interaction terms
3. Representation of random state, for replicability
4. Dedicated code path for shared weights ("buckets"), including support for a weights table indexed on secondary characteristics rather than obs/alt id's only

See also

PR #37: core sampling support
PR #32: distance bands

Issues #4, #5, #11: earlier discussions, closed
Issue #38: estimation corrections for non-uniform sampling
Issue #39: performance optimization