Setting NGHOST at runtime

[tomidakn]

Currently NGHOST is hard coded in defs.hpp.in as a constant fixed at cofiguration, but for higher order reconstruction this must be flexible. As the reconstruction order can be set at runtime, I think it is better to make NGHOST a normal variable initialized at the beginning of a simulation according to the reconstruction order and use of SMR/AMR.

Note that for SMR/AMR NGHOST must be even. For the third order reconstruction, it is OK to use NGHOST=3 on a uniform grid but it must be 4 with SMR/AMR because of the prolongation algorithm.