Add test dataset for two-station

> CDI Proposal Tier 1: Add a small test dataset with two stations of dissolved oxygen observations on which the new two-station estimation capabilities can be demonstrated and routinely tested.

Start by locating the repository's existing test datasets and tests for estimation capabilities, then compare their structure with the two-station dissolved oxygen observations requested here. Done means a small two-station dataset is available for demonstrating and routinely testing the new two-station estimation capabilities.

Written by the indexing model from the issue text.