Correct error in vertex-centred prolongation operators

CarpetLib: Some reformatting

CarpetLib: Correct error in vertex-centred prolongation operator

Adding a signed and an unsigned integer is always unsigned in C++, and a loop over the stencil terminated early for high order prolongation and small SIMD vector sizes. This has now been corrected.

I salute the designers of languages such as Julia (where such errors can be caught, and where array sizes are signed to prevent such headaches) or Haskell (where such automatic conversions do not occur).