Elemwiseopenmp

Hi,

I add openmp to elemwise operation. The case of contiguous tensors is quite simple, but for non contiguous case I had to rearrange the code so that the threads can't update pointers.

I also add a new configuration's variable, that I called openmp_minsize. The openmp is activated only for numpy vectors of "size" greater that this value, if openmp is activated.

I made some tests, and with openmp_minsize=200000 I get a 2x speed up for different operations