Exponential by tau approximation for itau object.

   [F, e, n] = expt(x)

   F = expt(x) is a function to approximate by tau method exp(x), x in 
   [a, b]. This function goal is to take the machine precision in this
   approximation, once expt(x) can be a funcional coeefficient at the
   equation. For this porpouse, we are using schursolver function, which
   will decide the approximation degree to achieve 1e-16. Concluding, now
   is possible to use expt(x) as coefficient.

   x = independent tau variable (itau object).

   F = polynomial approximation of exp(x) in [a, b] evaluated (with 
       orthovalM) in argument M = matrixM(x) (double matrix).

See also
  sint, sinht, cost, cosht, and logt.