Enable pattern matching which compares explicit index or object names

[kpeeters]

See https://cadabra.science/qa/2362/pattern-matching-noncommuting-expressions-matrices-indices : it would be useful to have a substitute conditional which lexicographically compares index or generic object names. So that you can write

substitute(ex, $\gamma^{a?} \gamma^{b?} | a? < b? -> \gamma^{b?}\gamma^{a?} + 2\eta^{a? b?}$);