Simple WCS elevation coverage construction

[pdavidc]

Child of #482

Given a service address and a coverage ID, enable simple construction a WorldWind elevation coverage for that coverage.

Here's a snippet demonstrating the concept. The structure passed to TiledElevationCoverage in this snippet is not necessarily what's we want, but intends to convey the rough idea:

var coverage = new TiledElevationCoverage({
    serviceAddress: "https://worldwind26.arc.nasa.gov/wcs",
    coverageId: "gebco"
});