Add more runtime NGHOST checks (especially for SMR/AMR)

[felker]

As I was adding a myriad of runtime checks to reconstruction.cpp for ensuring compatibility with the 4th order solver in #157, it occurred to me that there are few traps on the selected NGHOST value because it was assumed that NGHOST=2 for most of the history of Athena++. NGHOST was generalized to a compile-time parameter and configure.py flag --nghost in 5721084 on 2018-03-03 in order to support PPM and other high-order solver components.

Here are the following cases I have encountered or envisioned that could benefit from a safety check:

• For periodic boundary conditions,block_size.nx1 >= NGHOST/2 in all MeshBlocks regardless of the algorithm.
  • This has become an issue when using NGHOST=4 for PPM with MHD with small MeshBlocks with meshblock/nx2=6, for example.
• NGHOST % 2 == 0 when using SMR/AMR.
  • I was using NGHOST=3 minimum requirement for PPM for R-T instability simulations with AMR in Fix asymmetry of Coordinates x1f for multiple MeshBlock grids #112, but I believe that should have been forbidden / rounded up to NGHOST=4.
  • Actually, the restriction is stronger insofar that the ghost cell buffer size should be a power of 2 on all sides, right @tomidakn?
  • The Wiki documentation for SMR and AMR predate the NGHOST>2 change, so they should probably be updated.

In the interest of protecting the users from themselves, I think we should add these conditions to the Reconstruction class constructor (or somewhere else) and document the requirements in the Wiki.