Maryland Imagery Updates to 2016

[talllguy]

This is an update to #148 which added Maryland statewide six-inch imagery. The imagery now includes 2016 imagery for Howard County (true orthophotos) The description of the service is updated to reflect this change, as is the status page.

The data in the Maryland imagery layer now includes data from three years (2013, 2014, and 2016). It is my understanding that new data will be continuously loaded to this service as it becomes available. Therefore, consider removing the year from the imagery layer name.

[talllguy]

Note that the URL does not need adjustment, only the name of the layer and the metadata. Here's an example showing a new development in Howard County, Maryland, that would not have been visible in the 2013 imagery.

cc @osmlab/team-baltimore