[dtypes] odML custom tuples are not valid dtypes

[mpsonntag]

Currently the list of supported odML dtypes does not contain odML custom tuples. This has two consequences:

• odML style tuples can only be created, when creating a new Property, since the dtype is currently not checked during Property init. See also [Property] Init allows invalid dtypes #253. If this should change, odML custom tuples could no longer be created.
• the dtype of a property cannot be set to an odML custom tuple, since it is not in the list of supported odML dtypes.

Tuple creation example:

p = odml.Property(name="tuple", dtype="2-tuple", value="(12; 12)")

Tuple assignment fail example:

p = odml.Property(name="tupleFail", value="(12; 12)")
p.dtype = "2-tuple"