WIP: Reimplement core_cython routines as ufuncs

This eliminates the need for much of the explicit broadasting and
type checking.

At the moment, the patch just implements `healpix_to_lonlat_ring`.
I'd like to ask for feedback on the basic approach before
undertaking conversion of the other functions.

Fixes #72.

astropy_healpix/core_cython.pyx

@@ -19,6 +19,9 @@ npy_double = np.double
 npy_intp = np.intp
 npy_int64 = np.int64
 
+np.import_array()
+np.import_ufunc()
+
 
 def _validate_order(str order):
     # We also support upper-case, to support directly the values
@@ -32,6 +35,51 @@ def _validate_order(str order):
         raise ValueError("order must be 'nested' or 'ring'")
 
 
+@cython.boundscheck(False)
+cdef void healpix_to_lonlat_ring_loop(char** args, intp_t* dimensions,
+                                      intp_t* steps, void* data) nogil:
+    cdef intp_t n = dimensions[0]
+    cdef intp_t i
+    cdef int64_t xy_index, healpix_index, nside
+    cdef double dx, dy
+    cdef double* lon
+    cdef double* lat
+
+    dx = 0.5
+    dy = 0.5
+
+    for i in prange(n, nogil=True, schedule='static'):
+        healpix_index = (<int64_t*> &args[0][i * steps[0]])[0]
+        nside = (<int64_t*> &args[1][i * steps[1]])[0]
+        lon = <double*> &args[2][i * steps[2]]
+        lat = <double*> &args[3][i * steps[3]]
+        xy_index = healpixl_ring_to_xy(healpix_index, nside)
+        healpixl_to_radec(xy_index, nside, dx, dy, lon, lat)
+
+
+cdef void *no_data[1]
+cdef np.PyUFuncGenericFunction loop_funcs[1]
+cdef char types[4]
+no_data[:] = [NULL]
+loop_funcs[:] = [healpix_to_lonlat_ring_loop]
+types[:] = [np.NPY_INT64, np.NPY_INT64, np.NPY_DOUBLE, np.NPY_DOUBLE]
+
+
+healpix_to_lonlat_ring = np.PyUFunc_FromFuncAndData(
+    loop_funcs,
+    no_data,
+    types,
+    1, # number of supported input types
+    2, # number of input args
+    2, # number of output args
+    0, # `identity` element, never mind this
+    "healpix_to_lonlat_ring", # function name
+    "TODO", # docstring
+    0 # unused
+    )
+
+
+
 @cython.boundscheck(False)
 def healpix_to_lonlat(np.ndarray[int64_t, ndim=1, mode="c"] healpix_index,
                       int nside, str order):