- added Clifford algebra unity element

- added Clifford algebra unity element
- superfluous unity elements are removed from Clifford and color strings
- added Clifford contractions:
    gamma~mu*gamma.mu
    gamma~mu*gamma~alpha*gamma.mu
- added color contractions:
    d.aac
    d.abc*d.abc
    d.akl*d.bkl
    d.abc*f.abc
    d.akl*f.bkl
    f.abc*f.abc
    f.akl*f.bkl
- delta tensor and color structure constants can be evaluated numerically
- color_T(), color_d() and color_f() check their arguments
- added a couple of utility functions to idx.*
- simplify_indexed() doesn't crash any more when used on expressions containing
  noncommutative products