Recursive function used by QUAD1D3MCU2.

  [Q,cnt] = quadstp(F,a,b,tol,lev,fa,fc,fb,Q0) tries to

  approximate the integral of f(x) from a to b to within a

  relative error of tol.  F is a string containing the name

  of f.  The remaining arguments are generated by quad or by

  the recursion.  lev is the recursion level.

  fa = f(a). fc = f((a+b)/2). fb = f(b).

  Q0 is an approximate value of the integral.

  See also QUAD1D3MCU2 and QUAD8HSRNMC.