Revert "Try not going through the CIRS frame"

This reverts commit 0c0849c.

This makes things much slower, almost 10x. This is likely because the
scalar pole and field centre are broadcast against all antenna
positions and times.

montblanc/util/parallactic_angles.py

@@ -118,9 +118,13 @@ def _parallactic_angle_astropy(times, ap, fc):
     fc = SkyCoord(ra=fc[0], dec=fc[1], unit=units.rad, frame='fk5')
     pole = SkyCoord(ra=0, dec=90, unit=units.deg, frame='fk5')
 
+    cirs_frame = CIRS(obstime=times)
+    pole_cirs = pole.transform_to(cirs_frame)
+    fc_cirs = fc.transform_to(cirs_frame)
+
     altaz_frame = AltAz(location=ap[None,:], obstime=times[:,None])
-    pole_altaz = pole.transform_to(altaz_frame)
-    fc_altaz = fc.transform_to(altaz_frame)
+    pole_altaz = pole_cirs[:,None].transform_to(altaz_frame)
+    fc_altaz = fc_cirs[:,None].transform_to(altaz_frame)
     return fc_altaz.position_angle(pole_altaz)
 
 if __name__ == "__main__":