
A StringConverter is used to convert 
numbers into strings in a given format.
This conversion is done in the methods
'convert' or 'operator()'.  The format is
described by the attributes 
- range (numbers left to the separator)
- precision (numbers right to the separator)
- sign (bool)
- separator (char, usually point (American) or comma (European)).
These attributes are also used for a StringConverterDescriptor.