tauRecTools: improve CombinedP4FromRecoTaus

Hello,

This MR is trying to improve and clean up CombinedP4FromRecoTaus on a best-effort basis, although it could be further improved by avoiding code duplication (many functions are very similar and could be streamlined). Function arguments are now const references. Two weaknesses have been identified: the code will misbehave for 2-prong taus that we intend to support in R22, and for detector upgrade (hardcoded 2.7 |eta| cut). To be improved.

Note: systematically moving to const references was motivated by code readability, but may be sub-optimal in terms of performance. To be followed up.

Cheers, Bertrand