Recursive function used by QUAD .
[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 QUAD8 .