Implement pressure effects in the Todd-Longstaff mixing parameter

The Todd-Longstaff model is extended to incorporate pressure effects
The solvent viscosity is then caculated as

mu_eff = mu_s^(1-\alpha * \omega) * mu_mix^(\alpha * \omega)

where \omega accounts for the porous media effects and \alpha =
\alpha(pressure) accounts for the miscibility of the solvent and oil
when contacted.
The \alpha values can be given using the TLPMIXPA keyword

If no entries are given to TLPMIXPA the table specified using PMISC will
be used as default.
IF TLPMIXPA does not appear in the grid \alpha = 1 and the pressure
effect is neglected.
This is tested in test_solventprops_ad.cpp

opm/autodiff/SolventPropsAdFromDeck.hpp

@@ -143,6 +143,13 @@ public:
     /// return                      Array of n mixing paramters for density calculation
     V mixingParameterDensity(const Cells& cells) const;
 
+    /// Todd-Longstaff pressure dependent mixing parameter
+    /// \param[in]  So              Array of n oil fraction values. Soil / Sn values, where Sn = Sgas + Ssolvent + Soil.
+    /// \param[in]  cells           Array of n cell indices to be associated with the fraction values.
+    /// return                      Array of n pressure dependent mixing paramters
+    ADB pressureMixingParameter(const ADB& po,
+                              const Cells& cells) const;
+
 
 private:
     /// Makes ADB from table values
@@ -185,6 +192,7 @@ private:
     std::vector<NonuniformTableLinear<double> > pmisc_;
     std::vector<NonuniformTableLinear<double> > sorwmis_;
     std::vector<NonuniformTableLinear<double> > sgcwmis_;
+    std::vector<NonuniformTableLinear<double> > tlpmix_param_;
     std::vector<double> mix_param_viscosity_;
     std::vector<double> mix_param_density_;
 };