cet-is » fhicl » fhicl-cpp

This request is to add something of the like of:

std::optional<value_type> operator() () const

to fhicl::OptionalAtom, fhicl::OptionalSequence and fhicl::OptionalTable to return the optional value wrapped in a C++ optional data type.
This has the advantage of uniforming the interface with the non-optional parameters and to allow an easy initialization of data member from the configuration directly in the initialization list inside a constructor, with the added bonus of the ability to supply a default value via std::optional::value_or().