[MPL] solid grain pressure variable for density.

Used by exponential solid density.

MaterialLib/MPL/VariableType.cpp

@@ -48,6 +48,10 @@ Variable convertStringToVariable(std::string const& input)
     {
         return Variable::grain_compressibility;
     }
+    if (boost::iequals(input, "solid_grain_pressure"))
+    {
+        return Variable::solid_grain_pressure;
+    }
 
     OGS_FATAL(
         "The variable name '{:s}' does not correspond to any known variable",

MaterialLib/MPL/VariableType.h

@@ -52,6 +52,7 @@ enum class Variable : int
     liquid_saturation_rate,
     phase_pressure,
     porosity,
+    solid_grain_pressure,
     strain,
     stress,
     temperature,