Exponential for matrix for itau object.

   F = expm(x, coef)

   F = expm(x) returns the F matrix suth that F is the Taylor series
   expansion for function of matrices. Here is sused Malab's function 
   "funm" for our x propertie, it means, matrixM(x) = M.

Input (required)
   x    = independent tau variable (itau object).

Input (optional)
   coef = coefficients of a polynomial (double vector).
          If coef is informed, then we want exp(sum[coef_iP_i(M)]).

   F    = matrix sized by [x.n x.n], translating exp(M) in terms of
          its Taylor series expansion (double matrix in F.mat).

See also
  sinm, sinhm, cosm, coshm, logm and sqrtm.