Include .h files in distribution.
Fix some bugs having to do with interpolation for single knot.

* The GSL allocation routines require at least 2 points for linear,
  and at least 3 for spline.  The previous version assumed 1 point
  was okay for linear.  Now don't allocate unless there are 2 or more.
* Require 3 points for 1D model spline.
Rename getMeanFlux... to getFlux... since it isn't really the mean.
Backport of minor fixes from pofd_coverage.