Provide helper functions for two-station methods

> CDI Proposal Tier 1: Add helper functions to estimate ancillary variables relevant to two-station studies (e.g. reach-averaged light, travel times from the upstream to downstream stations).

Helper functions can support data preparation for light, travel times, and using the travel times to pair upstream and downstream observations. We could also include a helper function to construct a discharge-traveltime relationship based on conductivity pulses in paired sondes, if we expect that option to be common e.g. in tailwaters.