Support for modern compilers and C++17 plus some minor fixes

Koen Woltersrequested to merge
kwolters/allpix-squared:master into master

Few small updates, partly cherry-picked from !142 (closed), to support modern compilers and allow for a full C++17 build (if ROOT is build with C++17). Also contains a few minor other changes.

Should be quick to review.

Merge request reports