Refresh timestamp for multiapp to avoid concurrent requests on a fresh YAML each refreshing the service to no end.

Plus workaround 'tuple' error in _call__ (not a good hack)
Update multiapp.py

Updating fix as suggested by OLT.
Merge pull request #102 from keithamoss/multiapp-service-reinit

Avoid concurrent requests to uninitialised services triggering more than one initialisation