respect the fact that non-SI temperature may be offset from zero

also, the UnitTests test was buggy.

V2: improve the accuracy of the °F->K offset, unit test all temperature measures

opm/parser/eclipse/Units/UnitSystem.hpp

@@ -104,6 +104,7 @@ namespace Opm {
         std::string m_name;
         UnitType m_unittype;
         std::map< std::string , Dimension > m_dimensions;
+        const double* measure_table_to_si_offset;
         const double* measure_table_from_si;
         const double* measure_table_to_si;
         const char* const*  unit_name_table;