- Vignette --> ./inst/doc/other-Bessels.Rnw

- More docu in ./man/Bessel.Rd

- Add (most / all) of ~/R/MM/NUMERICS/bessel-fn.R
  R/I-fn.R  -- but we need to make these work with *complex* input
  ========   should not be hart, since the Abramowitz & Stegun formula
   are for complex .. though restricted, e.g. 9.7.1 (p.377) to  |Arg(z)| < pi/2
