Follow-up from "Correct weights loop in storeInitialWeights in CaloFutureShowerOverlapTool"

The following discussion from !1304 (merged) should be addressed:

• @graven started a discussion:

  Even nicer (IMO) would be to define a small, generic, helper function for this:

  #include <functional>
  #include <utility>
  
  namespace {
    template <typename... MapArgs, typename Predicate>
    void erase_if( std::map<MapArgs...>& map, Predicate&& predicate ) {
         auto i = map.begin();
         auto end = map.end(); // std::map::erase does not invalidate end -- so it can be cached
         while ( i != end ) {
             if (std::invoke(predicate,std::as_const(*i))) i = map.erase(i);
             else ++i;
         }
    }
  }

  and use it here as:

     erase_if( weights, [](const auto& w) { return w.second == 1. ; } // warning: floating point equality is unlikely to yield 'true'...

  This moves the 'boring details' of how to properly erase things from a map out of the body of the 'interesting' code, making it easier to read ;-)

  And once we get to C++20, one can use std::erase_if (see here) for this.