Improve interpolation weight computation code.

IceGrid::compute_point_neighbors() computes indexes of grid
columns (rows) both to the left (bottom) and to the right (top) relative
to a given point. (If we only compute the index of the column to the
left j, then j+1 may turn out to be invalid.)
Replace -Winline with -Wno-inline to avoid bogus warnings.

GCC warns about failing to inline default destructors. Weird.
Silence compiler warnings.
Fix IceModelVec::check_array_indices() (now it works for 3D fields too).