Add a new Target Solver which does not backpropagate changes to the upstream or does not do any fancy thing, i.e. just copies changes to simulation locally and propagate them to the downstream only by expanding the simulation. The behavior of this Target Solver should be taken as the most standard way.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@35365 20353a03-c40f-0410-a6d1-a30d3c3de9de