fix install of missing headers, added CMake-package, usage-examples

Features:

• introduced new interface-library libClaPP_SANITIZER and removed interface-library ClaPP_OPTIONS.
• introduced new cmake-files cmake/gsl.cmake and cmake/gtest.cmake.
• GSL and gtest are now included via CMake-find_package (and not via pkg_check_modules).
• introduced CMake-alias-library libClaPP::clapp.
• added support for CMake-package-install.
• added usage-examples (see folder usage).

Fixes:

• fixed install of missing parser_container.h*-headers.
• fixed pedantic-compiler flag-whitelist required for the GSL.

changed milestone to %v.0.5.0

assigned to @martin

approved this merge request

merged