Update HYDAT datums

[gdelaplante]

The AquaCache uses datums pulled directly from the HYDAT datums table on initial setup. This makes it easy to integrate our primary data sources, WSC timeseries, since they use the HYDAT datums. However, the datums in AquaCache are not currently being updated when a new version of HYDAT is incorporated by function update_hydat. This means that new datums published in HYDAT don't make their way to the datums table, but also that WSC locations don't get their updated datums.

Complete function update_hydat_datums to ensure up-to-date vertical datums. Notes are written in the function to facilitate this.