Fix conan create on Windows

See merge request !58

- _CRT_SECURE_NO_WARNINGS suppresses warnings for "unsafe" fopen, strcpy etc.
- int -> size_t

On Windows, the static library for the protobuf component has to be explicitly linked in the test_package. This addresse issue #103

Specifying 'all' is not necessary and does not work on Windows. On Windows the target is called BUILD_ALL.

Fix deprecations + updates

See merge request !57

See !92 for the relevant issue.

Resolve "Unit tests unter Windows"

See merge request !56

- Fix GTest spelling in test/CMakeLists.txt
- Restrict vcpkg triplet to release
- Add set(CMAKE_POSITION_INDEPENDENT_CODE ON)
- Include of <gtest/gtest.h> in tests instead of smaller headers
- Add boost options to CMakeLists.txt

This should be done by the Docker image

For activating the conan venv in ~/.bashrc

- Add test targets for both Debug and Release

boost::filesystem::load_string_file was removed in boost 1.84

See https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0174r2.html#2.1

- add scopes to enforce destruction of FileWriter, i.e. close file
- only check file size after writing the complete file

Something something string literals on Windows...