Bumping MCUtils version to 1.3.4 in 21.6

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.