Removed nogil block. The queue wont work w prange
Use Cython function pointer to separate kernels

Separated the data loop and redistretization into a cdef function and
pass a pointer to the kernel.
Control what field to benchmark
PEP8 fix on tesseroid test

Few changes to benchmark as well
Moved all fields to new Cython rediscretization

Got consistent speedups with respect to Tesseroids implementation. The
biggest gains are in gzz