WIP: try guvectorize

[albop]

The functions which evaluate at one single point, can be vectorized using guvectorize. This is currently much faster than repeated calls of the evaluation function and almost as efficient as a manually inlined vectorized call.
Also, one can take advantage of the new 'parallel' keyword, to use multiple cpus.
Here are the timings for the usual experiment: 50x50x50 grid. Evaluation on 10^6 points:

Manual vectorization : 0.1056051254272461
GUVectorize : 0.10982108116149902
GUVectorize (parallel): 0.03372311592102051

Advantages:

• for the same price: scalar evaluation and allocating/non-allocating calls.
  Caveats:
• ... ? (maybe, gufun signatures involving scalars is unnatural)