lisa.wlgen.rta: Fix a RecursionError
lisa.wlgen.rta: Fix a RecursionError

Fix possible RecursionError in RTAPhaseTree.children property on deeply
nested trees, such as the ones resulting from repeated accumulation
using + operator.

Computing the children when the object is created fixes the issue since
the recursion will always hit the memoization cache, avoiding an actual
deep recursion that would happen the first time the whole tree is
inspected.

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