Periodic boundary conditions

[paulromano]

There are a various stubs in the code for periodic boundary conditions, e.g., there is a BC_PERIODIC constant, but no implementation. Eventually it might be nice to have periodic boundary conditions in case someone wants to do a quarter-core model or the like.

[paulromano]

Axially-symmetric periodic boundary conditions were implemented in #647. I'll leave this issue open until we implement rotationally-symmetric periodic BCs.