src/include/clapp/option_container.h: introduced option_container_t (move|copy)-constructors (and deleted (move|copy)-assignment operators)

src/include/clapp/option_container.h: introduced basic_option_container_t (move|copy)-(constructors|assigment-operators)

src/include/clapp/option_container.hpp: throw exceptions, if optional options were registered on an xor-container

src/include/clapp/argument.h: changed value- and has_value-func to be not optional anymore (removed unused tests)