Switch to using atan-qd/taylor as the default atan implementation.

qd-fun.lisp:
o Use atan-qd/taylor and atan2-qd/taylor as the atan-qd and atan2-qd
  implementation.
o Fix bug in atan-qd/taylor. We need a special case for x small where
  we don't want to use the table. (Found by our rt-test.  Yay!)

rt-tests.lisp:
o Add comment for oct.atan.4 for why it works.