Do sum between models instead of using integrate

[SergeStinckwich]

We should be able to do a tensorial sum between two models, two produce a new, not using integrate: that modify a model to add another one. This is not a good idea to modify a model that could be used somewhere else.

Current situation:

model := KEModel new.
concernA := KEModelPart new.
concernB := KEModelPart new.
model integrate: concernA.
model integrate: concernB.

What would be better to have:

model := KEModel new.
concernA := KEModelPart new.
concernB := KEModelPart new.
model + concernA + concernB

After removing integrate:, we could simplify KEPopulation, by removing reset method that is needed by the current situation.