Friction theory and SF6 viscosity and tests

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
REFPROP backend can handle pseudo-pure fluids
HFC pseudo-pure mixtures viscosity

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
SF6 and H2S viscosity - still problems with H2S

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>