WIP: Bumping MCUtils version to 1.3.4 in 21.2
Release notes:
* Add isHiddenValley with PDG-standard (n,nR) check.
* Add PDG-standard (n,nR) = (0,0)|(5,9) digit check to isDarkMatter.
* Add PDG-standard nR = 0 digit check to isExcited.
* Add isHeavyFlavor canonical alias for isHeavyFlavour.
* Add isHadron||isQuark checks to hasDown,Up,etc. quark content functions and isHeavyFlavour.
* Add isBSM() checks to isLepton, isMeson, isHadron, isDiquark, etc. -- these should only return true for SM particles.
* Introduce preferred nucl{A,Z,Nlambda} preferred aliases, cf. Rivet.
* Fix Doxygen grouping markers.