The cnetdiff application has been refactored to be callable; Makefile tests...

The cnetdiff application has been refactored to be callable; Makefile tests have been removed and replaced by gtests. (#5323)

* The cnetedit applciation has been refactored to be callable; Makefile tests have been removed and replace by gtests. Addresses #5322.

* Moved CHANGELOG.md entry for cnetdiff changes from "Added" section to "Changed" section per reviewer suggestion. Addresses #5322.

* Reverted back to original ControlNetDiff source and header files per reviewer suggestion. Addresses #5322.

* Changes to cnetdiff.cpp and main.cpp to use the addLogGroup method to add PvlGroups to Pvl. Per reviewer suggestion. Addresses #5322.

* Put back line erroneously removed in CHANGELOG.md, per review. Addresses #5322.