Fix a build error using JDK7

A build error using JDK7 was uncovered by unit testing in module unsupported/coverage-experiment/netcdf. The fix is applied in module unsupported/coverage-experiment/unidata. The fix is to use an explicit Comparator for a TreeSet.
Throw ClassCastException when comparing incompatible ranges types
[GEOT-4523] dpi rescaling won't work on graphics that do not have a size
Merge pull request #237 from brett-walker/fix-unidata-jdk7

Fix a build error using JDK7