(from clang-gridtools #29) make_independent for Stages

[twicki]

Insert make_independent when applicable in the code generation for gridtools

[Stagno]

@twicki
I read from current GridTools documentation that the purpose of make_independent is to let 2 stages run in parallel.

If two stages have no dependencies, they don’t need to be synchronized. Thus, the user can make them independent.

But in dawn such stages would be merged together, right?
I don't know GridTools codegen enough to tell whether this matters or not.

[twicki]

Basically we don't need it since we can always fuse these stages. Make independent is just that in the GridTools world.