Depth below chart datum

Publishes the tide-compensated depth as environment.depth.belowChartDatum, measured from the tide station's chart datum, and names that datum in environment.tide.datum (#187).

• environment.tide.datum is the station's chart datum, such as MLLW, LAT or CD. The tide heights use it too.
• environment.depth.belowChartDatum is null for a station without a chart datum.
• The depth path has meters and the depth display category in its metadata.

Breaking: 2.4.0's environment.depth.belowSurfaceTideCompensated, belowSurfaceTideCompensatedLAT and belowSurfaceTideCompensatedMLLW are removed. Read environment.depth.belowChartDatum and environment.tide.datum instead.

Validation: lint, build, Node tests, and browser tests passed in CI.