tweaked to_string for TK attributes classes
first polymorphic try
that wasn't a good idea
new TKAttributeContainer; improved TKBaseAttribute

TKAttributeContainer is a general-purpose container for TKAttributes,
which allows for easily passing them along. Additionally, each
TKAttribute now identifies itself through a type defined in TKAttrType.
refilled t_mkvector
t_nested_mkvector now uses tkattributecontainer
small change to tkattributecontainer tostring