vectorize CarpetInterp2's fasterp interpolation

[rhaas80]

@rhaas80 's pull request 33 https://bitbucket.org/eschnett/carpet/pull-requests/33

• CarpetInterp2: use thorn Vectors for vectorization
• CarpetInterp2: remove buffer array

[rhaas80]

@ianhinder says

I seem to remember working on this code to ensure it was vectorised by the compiler. What motivated the explicit vectorisation?

[rhaas80]

Good question. The branch is rather old, so I do not quite remember anymore. I just saw that there was some code to enable vectorization by the compiler. I will conduct some benchmarking and disassembly to see how well the compiler (some new version of gcc and icc on Stampede2 I guess) does.