Fix atan/taylor bug and add atan2/taylor.

qd-fun.lisp:
o Fix issue with atan-qd/taylor for large args. We use the
  relationship atan(x) = pi/2-atan(1/x) for x > 1.
o Add atan2-qd/taylor.  This appears to be much faster than
  atan2-qd/newton. 

timing.lisp:
o Add timing test for atan2-qd.