Add multi-threaded logic for compressing data about gridfour

[gwlucastrig]

Just pushed code changes for this issue. The option for multi-threaded processing was introduced to the PackageData demonstration class. Additionally, the code base now includes a new class called TaskGroupExecutor that coordinates the use multiple threads when partitioning processing into sub-tasks. In some cases, this class is easier to use than the Java standard API's CyclicBarrier and Phaser classes.

In testing with the GEBCO global elevation and bathymetry data set (3.7 billion sample points), the time to compress the entire data set on a mid-sized laptop computer was reduced from 1920 seconds to 1052.

Opportunities for future improvements are being investigated.

[gwlucastrig]

Final changes pushed to repository. These enhancements resulted in a nearly 50 % reduction in processing time for global elevation/bathymetry data sets.