Refactored object model of Sb/Pws classes

They all now derive from common base classes rather than Pws from Sb.
Affected classes:

SbConstraint
* PwsConstraint
SbContainer
* PwsContainer
SbTsoBwd
* PwsPsoBwd

are now

ChannelConstraint
* SbConstraint
* PwsConstraint
ChannelContainer
* PwsContainer
ChannelBwd
* SbTsoBwd
* PwsPsoBwd