Add a 'robust mode': A parse error during interpretation of a segment will be turned...

Add a 'robust mode': A parse error during interpretation of a segment will be turned into a Warning, and a generic object will be created.
Should guarantee that no exceptions are triggered during communication, even if there's a bug in the segment definitions. Downside: find_segments('FOO') might return instances that are not of subclass FOOx.
Compensation: Make all calls to find_segment type safe by requiring a specific subclass.