Release 0.3.0 including non-param-options behavior change

Release 0.3.0:

This release uses (nearly everywhere) the same interfaces (as in 0.2.5) but changed the implementation under the hood. These changes are a preparation for future features of libClaPP (where options and parameters can be combined logically and grouped together).

One of these exceptions where the behavior changed are non-param-options (i.e. basic_option_t), which were modified slightly. Just search for basic_option_t in the changes below.

Release 0.3.0 contains the following changes:

• added support for leak sanitzier.
• new Gitlab-CI-pipelines (added the following new architectures and OSs):
  • includes amd64 (debian 10, ubuntu 20.04, ubuntu 21.04)
  • includes arm64 (debian 10, ubuntu 20.04, ubuntu 21.04)
  • includes armhf (debian 10)
• added support for clang- and gcc-compilers up to version 12 (however, C++17 is still a mandatory requirement).
• moved clang-tidy build-options to cmake/clang_tidy.cmake (including a transition to clang-tidy-12 and several code-fixes newer clang-tidy-versions complained about).
• moved compiler build-options to cmake/compiler_flags.cmake.
• bump clang-format-7 to clang-format-11 (as clang-format-7 formatted code leads to compound-token-split-by-space warning in clang-12).
• bump submodule third_party/googletest to version 1.11.0.
• bump submodule third_party/GSL to version 3.1.0.
• fixed documentation.
• fixed and added new tests.
• renamed test-names from snake-case to camel-case. (as suggested by google-test framework documentation)
• updated help text generation.
• type of basic_option_t::_value changed from std::optional<T> to T.
• operator bool() of an basic_option_t now returns _value != default_value instead of the value returned by has_value().