Remove unnecessary double diffusion parameter

For some reason there was a temperature diffusion parameter in the salt
fingering regime while Large et al. (and the current POP implementation, based
off of Gokhan et al.) set the temperature coefficient to 0.7*salt coefficient.
I removed this parameter and cleaned up the stand-alone driver for double
diffusion (I also added the salt diffusivity coeffients to the output and
updated how the NCL script generates the salt regime plot, though the actual
graphic looks the same as before)
Update README file, clean up some tests

The README file was falling out of date, so I made sure it was up to date. In
doing so I realized some of the PDFs produced by NCL scripts in the regression
test directory had unclear file names so I updated a few of those scripts.

I don't think I'm clear on the netCDF description attribute for the Tdiff
variable - sometimes I call it "temperature diffusivity" and sometimes I call
it "tracer diffusivity." I should probably clear that up at some point...