Allow MoabMeshTransfer to support OpenMC as the main app

For coupling between Aurora and time-dependent multi-apps, it can be advantageous to allow OpenMC to run on the coarsest time grid by using the sub-cycling system. This requires OpenMC to be the main application, but the `MoabMeshTransfer` currently only supports OpenMC being the sub application. 

We need to support the other `direction` in the `transfer()` method.