3eqn melting parameterization according to Hellmer & Olbers as in https://github.com/matthiasmengel/pism/commits/dev_ocean_th by Matthias Mengel
added POGivenTH files
update unpublished reference
Use gsl-config to find GSL.
Add a forgotten if(...).
merge 878d17ee2ad1e772; fix trivial conflict in doc/ice_bib.bib