pfcalc 0.6 released

Changes from version 0.5:
* Implemented variation of fluid density with temperature. Changed liq file format. Every line contains 3 comma separated values: temperature, density and viscosity.
* Removed built-in water properties. Now only default values for temperature, density and viscosity are built-in. The water definition has been moved to a liq file.
* Changed data structures for fluid representation.
* Implemented sorting of values from liq files by temperature, using qsort.