[Property] Exception on x-tuple Property extend or append

When trying to append or extend an x-tuple Property value, a ValueError occurs.

e.g.

```
prop = odml.Property(name="tuple-test")
prop.dtype = "3-tuple"
prop.values = "(7; 8; 9)"
prop.values = ["(1; 2; 3)", "(4; 5; 6)"]       # reassignment works
prop.append("(7; 8; 9)")                          # append fails
prop.extend(["(7; 8; 9)", "(10; 11; 12)"])  # extend fails
```
