[Property] Init allows invalid dtypes

[mpsonntag]

Currently Property.__init__ does not check odML dtypes. The following example currently would work:

p = odml.Property(name="test", value=[1, 2, 3], dtype="someText")

When assigning the dtype of an existing Property, only supported dtypes are allowed. The following would raise an AttributeError:

p.dtype = "someText"

[mpsonntag]

Since a failed check for dtype raises an Exception, adding a dtype check to Property.__init__ would currently lead to an uncaught Exception in any of the parsers if an unsupported dtype is encountered.
Therefore the dtype in Property.__init__ will be handled as described:

1. Check a provided dtype.
2. If the dtype is unsupported and a value has been provided, use the value to set the dtype.
3. If no value has been provided, use 'string' as dtype instead.
   In both cases 2) and 3) provide a warning message to the user.