Add Wrapper Tool for std::transform

While in general it is a nice feat, in some instances it is a bit annoying that `std::transform' acts on its input string.

This MR adds a method to the allpix namespace that transforms a string and copies it, i.e.

std::string allpix::transform(const std::string& str, const T& op);

where op could be e.g. ::tolower.