1. need to add a test for the akima splines

2. [DOCUMENTATION] From: Conrad Curry <cwcurry@orca.st.usm.edu>

  I would suggest adding more about cspline and Akima similiar to what
  is given for 'polynomial', particularly under what conditions one
  would be prefered over the other.

3. add akima splines without the Wodicka modification, so that the spline
is a smooth curve without corners.