Fail textual import with AssignActionUsage

[adaussy]

The current implementation fails to import AssignActionUsage.

For exemple importing :

part def Counter {
	attribute count : ScalarValues::Integer := 0;
	
	action incr {
		assign count := count + 1;
	}
	
	action decr {
		assign count := count - 1;
	}
}

Ends up with :

 part def Counter {
        attribute count : ScalarValues::Integer := 0;
        action incr {
            assign :=;
        }
        action decr {
            assign :=;
        }
    }

This is due to the current implementation of that give an AST that deal with parameter node in way that is not fully compliant with the BNF.