Create unit tests, preparing to debug cross-slice pdb16 difference bug.
Add larger test images for debugging PBD16 read boundary problem.
Create difference value cache for future use in fixing PBD16 read boundary problem.
Fixed read-length related problem with PBD16 reader, by caching runs of difference values for next time.
 Refactor Pbd16InputStream to read one 16-bit short at a time.
Allow two-byte short value reads to be split across two separate read() calls.
Create Vaa3dPbdToRaw runnable class to test conversion from vaa3dpbd to vaa3draw format, to test recent code fixes.
Merge pull request #1 from cmbruns/master

Improve handling of PBD compression difference runs that span multiple read() calls.