Resolve "State key char instead of string"

Closes #352 (closed)

test/gtest_processor_motion.cpp

@@ -120,13 +120,13 @@ TEST_F(ProcessorMotion_test, getState_structure)
         WOLF_DEBUG("t: ", t, "  x: ", problem->getState().vector("PO").transpose());
     }
 
-    ASSERT_TRUE (processor->getState('P').count('P'));
-    ASSERT_FALSE(processor->getState('P').count('O'));
-    ASSERT_FALSE(processor->getState('O').count('P'));
-    ASSERT_TRUE (processor->getState('O').count('O'));
+    ASSERT_TRUE (processor->getState("P").count('P'));
+    ASSERT_FALSE(processor->getState("P").count('O'));
+    ASSERT_FALSE(processor->getState("O").count('P'));
+    ASSERT_TRUE (processor->getState("O").count('O'));
 
     WOLF_DEBUG("processor->getState(\"V\") = ", processor->getState('V'));
-    ASSERT_EQ   (processor->getState('V').size(), 0);
+    ASSERT_EQ   (processor->getState("V").size(), 0);
 }