[dtypes] odML style tuples are not properly saved

[mpsonntag]

When saving a Document containing Properties with odML style tuples, only the very first tuple is saved, the other ones are discarded.

The following example will save only the value (1; 2) to file:

doc = odml.Document()
sec = doc.create_section(name="tuple_test")
prop = sec.create_property(name="tuple_test", dtype="2-tuple")
prop.values = ["(1; 2)", "(3; 4)", "(5; 6)"]
odml.save(doc, './test.odml')

[mpsonntag]

Closing this issue also includes adding corresponding tests.