add THERMEXR field property

this is the thermal expansion ratio for each cell in a mechanics model
write this to the .INIT file for relevant models

opm/input/eclipse/EclipseState/Grid/FieldProps.hpp

@@ -151,6 +151,7 @@ static const std::unordered_map<std::string, keyword_info<double>> double_keywor
                                                                                       {"PRATIO", keyword_info<double>{}.unit_string("1")},
                                                                                       {"BIOTCOEF", keyword_info<double>{}.unit_string("1")},
                                                                                       {"POELCOEF", keyword_info<double>{}.unit_string("1")},
+                                                                                      {"THERMEXR", keyword_info<double>{}.unit_string("1/AbsoluteTemperature")},
                                                                                       {"MULTX",   keyword_info<double>{}.init(1.0).mult(true)},
                                                                                       {"MULTX-",  keyword_info<double>{}.init(1.0).mult(true)},
                                                                                       {"MULTY",   keyword_info<double>{}.init(1.0).mult(true)},

src/opm/input/eclipse/share/keywords/900_OPM/T/THERMEXR

@@ -0,0 +1,11 @@
+{
+  "name": "THERMEXR",
+  "sections": [
+    "GRID"
+  ],
+  "description": "The THERMEXR item is used to set the thermal expansion ratio for a cell in mechanics models.",
+  "data": {
+    "value_type": "DOUBLE",
+    "dimension": "1/Temperature"
+  }
+}

src/opm/input/eclipse/share/keywords/keyword_list.cmake

@@ -1154,6 +1154,7 @@ set( keywords
      900_OPM/S/STRESSEQUILNUM
      900_OPM/S/SUREA
      900_OPM/S/SWOFLET
+     900_OPM/T/THERMEXR
      900_OPM/T/TLPMIXPA
      900_OPM/V/VAPWAT
      900_OPM/Y/YMODULE

src/opm/output/eclipse/WriteInit.cpp

@@ -447,6 +447,7 @@ namespace {
             {"PERMZ"    , ::Opm::UnitSystem::measure::permeability},
             {"POELCOEF" , ::Opm::UnitSystem::measure::identity},
             {"PRATIO"   , ::Opm::UnitSystem::measure::identity},
+            {"THERMEXR" , ::Opm::UnitSystem::measure::identity}, // 1/(temperature difference)
             {"YMODULE"  , ::Opm::UnitSystem::measure::ymodule},
         };