Update stand-alone KPP driver

Improved output from test 4 (computing OBL depth above / below center of
level); test now prints out the diffusivities computed in the two columns. I
also swapped tests 4a and 4b: 4a is now "boundary layer depth above cell
center" and 4b is now "boundary layer below cell center." Next step will be to
include max_nlev for this test.