Add "String symbolic" filed in "MatrixValue"
Also, add getSymbolic() method in super interface "Value", and implement getSymbolic() in "MatrixValue",
then, "SimpleMatrixValue", "BasicMatrixValue" and "AdvancedMatrixValue" can all have this method; Moreover,
also implement getSymbolic() in "CellValue", "StructValue" and "FunctionHandleValue", but return null, may
reimplement them later when we try to handle Cell, Struct and FunctionHandle.