New ufuncs to add...

[WarrenWeckesser]

Some ideas for ufuncs and gufuncs to add to ufunclab.

• fsum: sum floating point values while retaining as much precision as possible in the result. See, e.g. numpy.sum not stable enough sometimes (Kahan, math.fsum) numpy/numpy#8786.
  Note: kbnsum, a gufunc that computes the Kahan-Babuska-Neumaier summation, was added in c770a45.
• barycentric_coords(x, simplex): a gufunc with shape signature that is effectively (n), (n + 1, n) -> (n + 1). x is a point in n-dimensional space, and simplex is a simplex (i.e. n+1 points) in that space.
• Lehmer mean. Naive implementation is easy. Robust implementation that avoids potential overflow or underflow in intermediate calculations requires a bit more work.

Done:

• ordered: check that a sequence is increasing, strictly increasing, etc.