Proposed enhancement for raster coverage elevation dimension default value.  

Proposed enhancement:
Add ability to specify the elevation default value in raster coverage so that GeoServer can use that default value if provided.  If not provided, GeoServer defaults to current behavior, it uses the minimum value in the extent.

Rationale:  
A customer wants the default elevation to be closest to the earth's surface.  GeoServer's existing default is the lowest numeric value.  This is generally closest to the surface, and for air isobaric level, the lowest numeric value is furthest from the surface.  Adopting this change will allow users to choose their elevation default values.

Dependencies:
relies on GeoTools change to add hints property to opengis project's org.opengis.parameter.GeneralParameterValue.java and referencing project's org.geotools.parameter.AbstractParameter.java 
(https://github.com/geotools/geotools/pull/305)