fine-tune setChanged (the real fix will be to store the _changed attribute for each client in the parameter's _clients map)

better setChanged strategy (the last change would break for parameters shared by several solvers, in a loop)

- if no client is given, set the changed flag for all the parameters;
- if a client name is given and "changed" is false, affect only the
 parameters that are owned exclusively by the client;
- if "changed" is true, affect all parameters that depend on this client

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