- color/clifford objects have representation label to distinguish elements

- color/clifford objects have representation label to distinguish elements
  of different algebras; objects with different labels commute with each
  other
- dirac_one() -> dirac_ONE()
- added gamma5 clifford objects which is constructed by dirac_gamma5()
- clifford::simplify_ncmul() anticommutes gamma5's to the front and removes
  squares of gamma5
- the argument to collect() can be a list of objects in which case the
  result is either a recursively collected polynomial, or a polynomial in
  a distributed form with terms like coeff*x1^e1*...*xn^en, as specified by
  the second argument to collect(). For example (ginsh):
    > f=a*x-x+a*x*y+x*y+a*x*y^2-2*x*y^2+y^2;
    a*x+y*a*x+y^2*a*x+y^2-x+y*x-2*y^2*x
    > collect(f,x);
    (-1+y+y^2*a+y*a+a-2*y^2)*x+y^2
    > collect(f,y);
    a*x-x+y^2*(1+a*x-2*x)+y*(a*x+x)
    > collect(f,[x,y]);
    (-1+y*(1+a)+y^2*(-2+a)+a)*x+y^2
    > collect(f,[y,x]);
    (-1+a)*x+y*(1+a)*x+(1+(-2+a)*x)*y^2
    > collect_distributed(f,[x,y]);
    (-1+a)*x+y^2*(-2+a)*x+y*(1+a)*x+y^2