- added parameter "incremental" to ::xotcl::Attribute: when set, one can use "object paramname add|delete $value" etc. - use setters with parameter constraints in slot optimizer - as a consequence, setting attributes via slot names is about twice as fast as before, when parameter constraints are used. - extended regression test
- made error messages for failed conversions more consistent (note that tcl does not provide the parameter name in the error message, but user-defined converters do)