Slowly migrating away from is_valid_linkage so that specific messages  are thrown, exception throwing code is not repeated, etc. Added warning  facility that works regardless whether the warnings module is available.
Slowly migrating away from is_valid_linkage so that specific messages 
are thrown, exception throwing code is not repeated, etc. Added warning 
facility that works regardless whether the warnings module is available.


deads/scipy-cluster,deads/scipy-cluster,krrepo/scipy-cluster,tectronics/scipy-cluster,krrepo/scipy-cluster,LogicalKnight/scipy-cluster,ozcanesen/scipy-cluster,krrepo/scipy-cluster,ozcanesen/scipy-cluster,tectronics/scipy-cluster,ozcanesen/scipy-cluster,deads/scipy-cluster,LogicalKnight/scipy-cluster,deads/scipy-cluster,krrepo/scipy-cluster,ozcanesen/scipy-cluster,tectronics/scipy-cluster,tectronics/scipy-cluster,LogicalKnight/scipy-cluster,LogicalKnight/scipy-cluster