ENH: fitpack2: add check_finite optional argument to UnivariateSpline

If an input array contains nans, FITPACK silently returns all
spline coefficients as nans. Better bail out early with a
sensible error message.
DOC: add a nan masking example to the UnivariateSpline docstring

example is by @juliantaylor,
https://github.com/scipy/scipy/issues/3531#issuecomment-40233093
DOC: tweak the docstring of UnivariateSpline