A logic issue existed when validating whether a `cl::opt<T>`
was set. This has been addressed by replacing `isDefaultOption`
method with `getNumOccurrences`, as the former one was not
meant to be used for checking if the option was set or not.
Likewise, the default Option itself is written to a `std::string`,
in method `get`; the latter is leveraged by `Invokable`.
Giacomo Vercesi