Reduce likelihood of shadowing symbols in OPM_THROW

The OPM_THROW macro uses an internal symbol, 'oss', which is, strictly
speaking, in the client's namespace.  Moreover, as the
"EclipseGridParseHelpers.hpp" header uses the same symbol, and for
similar purposes, this produces warnings at "-Wshadow" in GCC when
compiling that header.

Rename the macro's symbol to 'oss__' (double trailing underscore) to
reduce the likelihood of shadowing a previous definition.

opm/core/utility/ErrorMacros.hpp

@@ -48,12 +48,12 @@
 // std::runtime_error.
 //
 // Usage: OPM_THROW(ExceptionClass, "Error message " << value);
-#define OPM_THROW(Exception, message)                            \
+#define OPM_THROW(Exception, message)                                   \
     do {                                                                \
-        std::ostringstream oss;                                         \
-        oss << "[" << __FILE__ << ":" << __LINE__ << "] " << message;   \
+        std::ostringstream oss__;                                       \
+        oss__ << "[" << __FILE__ << ":" << __LINE__ << "] " << message; \
         OPM_MESSAGE(message);                                           \
-        throw Exception(oss.str());                                     \
+        throw Exception(oss__.str());                                   \
     } while (false)
 
 // throw an exception if a condition is true