tool_app_template::run() calls get_selected_operation() to obtain the
operation (command) the user selected. To do this,
get_selected_operation() does a CLI pre-parsing pass, with a minimal
boost::program_options, so things like mixed positional/non-positional
args are correctly handled. This code use `sstring` for generic
operation-options. The problem is that boost doesn't allow values with
spaces inside for non-std::string types. This therefore prevents such
values from being used for any option downstream, because parsing would
fail at this stage. Change the type to std::string to solve this
problem.