emplace doesn't overwrite already existing collection, so this would lead to leaking memory

Also, don't add particle as daughter to its parents.

This addresses an issue with fully duplicated entries in the parents and
daughters relations for MCParticles. After this bugfix, only a single
entry for each parents is entered in the parents relations, similar for
daughters. More demonstrated behavior in pull request comments.

BEGINRELEASENOTES
- edm4hep: don't add particle as parent to its daughters, and vice versa
ENDRELEASENOTES

This doesn't happen automatically in EDM4hep but does in LCIO!

 - need the units in namespace CLHEP