Fix #2 Preserve unsigned 64-bit decoded values

dbc_decoder.h

@@ -14,6 +14,7 @@
 
 #include <vector>
 #include <cstdint>
+#include <variant>
 
 #include "decode_database.h"
 
@@ -37,13 +38,13 @@ struct RawCanFrame {
  */
 struct DecodedSignalValue {
   const DecodeSignal *definition; /**< Signal definition. */
-  std::int64_t rawValue;          /**< Extracted raw integer value. */
+  std::variant<std::int64_t, std::uint64_t> rawValue; /**< Exact signed or unsigned raw value. */
   double physicalValue;           /**< Converted physical value. */
   bool valid;                     /**< true if decoding succeeded. */
 
   DecodedSignalValue()
     : definition (nullptr)
-    , rawValue (0)
+    , rawValue (std::uint64_t (0U))
     , physicalValue (0.0)
     , valid (false) {
   }