Fix timedelta scalar multiplication

This fixes a bug in elementwiseScalarMultiplyTimeDelta introduced in !5 (merged): in the original implementation, we had something like scaled_td += lambda * base_td but f3844207 replaced it with something like scaled_td = lambda * base_td.

The bug was the reason why we urgently had to introduce d7586b2b into the master right before the first open-source release of ICON (several Buildbot tests on the DWD machine kept failing). That commit was dropped and the master was updated with force-push.