lisa.wlgen.rta: Fix __add__ implementation
lisa.wlgen.rta: Fix __add__ implementation

Do not modify the object inplace for the regular addition, since the left
operand might very well be reused, and is not supposed to be modified by the
addition.

Add __iadd__ implementation if inplace modification is needed.

credp/lisa,ARM-software/lisa,credp/lisa,credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,ARM-software/lisa