Move the coverage implementation in its module

The grid coverage implementation currently lives in the main module. However, current implementation is almost completly independent of Feature, Filter and other services provided in main. Coverage could be splitted in its own module.

• It doesn't hurt Jody's plan to create a module for interfaces, since coverages are implemented on top of GeoAPI interfaces only.
• It will help developpers to isolate their work on grid coverage implementation.
• It will help to remove the JAI dependency from the main module.

The root packages to move are:

• org.geotools.coverage
• org.geotools.image