- replaced the Derivative() function by a more resonable fderivative class;

- replaced the Derivative() function by a more resonable fderivative class;
  to see it in action, try "series(abs(x),x==0,3)" and "series(abs(x),x==y,3)"
  in ginsh with previous GiNaC releases and with this one
- add::eval() throws an exception when the first term is non-commutative and
  the overall_coeff is non-zero
- small fixes to print_tree output of indexed objects
- ncmul::expand() now actually does expand something like "(dirac_gamma(mu)+1)
  *dirac_gamma(nu)" correctly (but the change to add::eval() should make this
  a "can't happen" case)
- moved ToString() to tostring.h (fewer header dependencies)
- ALL YOUR "BASES CLASSES" ARE BELONG TO US