gh-38: release Python GIL in numpy loops (#39)

The Python GIL is now released while numpy evaluates the inner loops of
the vectorised functions. This might help in applications where other
Python code runs in parallel to healpix carrying out large computations.

Closes: #38

python/chealpix.c

@@ -91,11 +91,14 @@ PyObject* vectorize(vecfunc func, void* args, npy_intp nin, npy_intp nout,
         npy_intp *sizeptr = NpyIter_GetInnerLoopSizePtr(iter);
         npy_intp *stride = NpyIter_GetInnerStrideArray(iter);
         char **data = NpyIter_GetDataPtrArray(iter);
+        NPY_BEGIN_THREADS_DEF
         if (!iternext)
             goto iterfail;
+        NPY_BEGIN_THREADS_THRESHOLDED(NpyIter_GetIterSize(iter))
         do {
             func(args, *sizeptr, data, stride);
         } while (iternext(iter));
+        NPY_END_THREADS
     }
 
     out = &NpyIter_GetOperandArray(iter)[nin];