stub for optional unit-testing with CUnit
initial unit-tests for cpt.c
fix in greyscale interpolation
Revert "fix in greyscale interpolation"

This reverts commit 1c2bd44e087cbcd5fb8c694932bffcf8854becc2.
clean up unit directory
clearer reporting in unit test top-level
fix for greyscale interpolation
per-unittest helper convention
tests now use helper functions, added rgb zfill test
tidying
moved fill helper-functions to their own file
stub for fill unit-tests
test-name simplification