- introduce .functional_integral() methods to both NearestLeafTree and
  HorizonHistories classes
- reimplement .weighted_mean() as a wrapper of .functional_integral().
  this results in a performance penalty since it must invoke
  .functional_integral() twice but avoids the code duplication that would
  result otherwise
- allow 0/0 in .weighted_mean().  if the numerator of the mean is
  identically 0 and the weighted denominator has also evaluated to 0 then
  return 0 instead of reporting a divide-by-0 error.  this is done
  presuming that this is caused by the weight function being defined in a
  way that causes it to go to 0 when the data being averaged goes to 0 and
  that if a proper limit of the ratio was evaluated it would be found to be
  0.