Avoid array temporaries

[matt-frey]

Fortran creates temporary arrays when passing sub-ranges to subroutines.

[sjboeing]

Thanks for writing this Matthias! I can see why you initially went for the other approach, but thought it would be worth trying to get rid of the compiler warnings. Not quite sure whether to do anything (by default) with the values of the output fields at z=-1 and z=nz+1.

[matt-frey]

We could also get rid of these warnings by explicitly creating a temporary variable ourselves.

[sjboeing]

What do you think is best practice/most efficient? I would be OK with either solution. Guess this is the next one to merge after the period BCs PR.

[matt-frey]

Avoiding temporary arrays is definitely more efficient as we do not copy memory. The indices -1 and nz+1 for these arrays are never used and we can either set them to zero or (maybe better) use linear extrapolation.

[matt-frey]

My suggested change: 5759bce

[sjboeing]

Let's use the extrapolation, which I agree should be safer.