* Fixed implementation of polyploid (het) compression in RR.
* The test for a usable site was all wrong. Worked out details with Mauricio to get it right.
* Added comprehensive unit tests in HeaderElement class to make sure this is done right.
* Still need to add tests for the actual polyploid compression.
* No longer allow non-diploid het compression; I don't want to test/handle it, do you?
* Added nearly full coverage of tests for the BaseCounts class.