Fixing the call of initialize() for InternalSideUserObjects (closes #2695)

Internal side user objects are block restricted now, so the initilize()
has to be called in a loop over the domain IDs, so that all objects are
correctly initialized.
Merge pull request #2696 from andrsd/fix-intialize-of-internal-side-user-objects

Fixing the call of initialize() for InternalSideUserObjects (closes #269...