MPI halo too large w.r.t. core region should trigger a warning?

```
from devito import *

grid = Grid(shape=(3, 3))

f = TimeFunction(name='f', grid=grid, save=5)

eq = Eq(f.forward, f + 1., subdomain=grid.interior)

op = Operator(eq)
op(time_M=3)
```

will produce wrong values on the two right MPI ranks in a 2x2 MPI grid (ie 4 MPI processes in total)

Probably because the halo is too large w.r.t. the core region

