BUG: special: reimplement struve functions in C
GEN: special: regenerate files
special/struve: fix negative z
special/struve: take underflowing bessel functions into account in bessel series error estimate
special/struve: try to postpone under/overflows in power series
special/struve: fix struve L overflow handling
special/struve: deal with breakdown of the asymptotic expansion
special/struve: fix convergence criterion of power series

For v < 0, |v| >> 1, the summand may start to increase again for n > v,
so push the convergence point far enough.
special/struve: update comments
special/struve: fix c++ style comments
Merge pull request #2712 from pv/special-struve

special: Reimplement Struve functions