[Users] New thorn PeriodicCarpet

We just added a new thorn PeriodicCarpet to the Einstein Toolkit. Unsurprisingly, this thorn provides periodic boundary conditions for simulations that use Carpet. PeriodicCarpet is implemented using Carpet's communication infrastructure, and is thus significantly more flexible and efficient than the existing thorn Periodic. In particular, simulations with irregular grid structures or with refined regions near or crossing the periodic boundaries will benefit from PeriodicCarpet.

-erik