Added files for docs for mixtures

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Added reducing parameters from GERG 2012 to JSON code

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Fixed get_global_param_string and get_fluid_param_string (wasn't returning the value)

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Added some tables from Kunz and Wagner

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Added code to parse table from Kunz and Wagner (GERG 2008) for excess terms

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Mixtures can now use JSON to load departure data, reducing data to follow in short order; some work on PPF fitting (still not working entirely)