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

- added toc
- fixed typo in outlook section
- fixed typo in subparser section
- fixed typo in sub-parser section
- fixed formatting of option-documentation
- fixed description of sub-parser example
- Clarified introduction of sub-parsers.
- clarify default value
- clarification of subparser section
- added two newlines in description of purpose

furhtermore, several other improvements were done:
- added new compiler warnings for gcc-10 and gcc-11 for pedantic builds
- created new build-targets (i.e. libClaPP_CXX_COMPILE_OPTIONS) which contain
  all warnings for the current compiler.

- bump version to 0.2.0
- updated README.md
- added and updated several tests
- added and updated doc
- updated sample_extractor and added sample_executor
- added clang-format specification
- added support for sanitizers
- added more exhaustive gitlab-pipeline
- added more specific exceptions
- updated help-message generation in parsers (sub_parser, main_parser and basic_parser)
- minor improvements of CMakeLists.txt (e.g. added cppcheck)
- renamed full_example to large_example
- minor improvements to examples
- added test examples scripts
- added build_info_string
- unified arguments and options

- new exceptions based on clapp::exception::clapp_exception.
- added `parse_and_validate()` function.
- added found-func
- added help-option
- added more constexpr
- added more comments to examples
- code cleanup
- bump version t0 0.1.0
- added gitlab-ci pipeline
- added double/float values
- added support for vector of short/long options
- added doc-extractor