#1604 put CLI11 in vt namespace

[jstrzebonski]

Fixes #1604

---

I didn't upgrade CLI11 to the newest version, API changed a bit. If we want to do so, let's open another issue for that. Also script generating single header file was changed, and I opened issue in CLI11 repo reporting problem with support for custom namespace - before upgrading we want it to be fixed

CLIUtils/CLI11#680

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FAILED: tests/collection_extended 
: && /usr/bin/g++ -O3 -DNDEBUG --coverage -rdynamic -Wl,-rpath -Wl,/usr/local/lib -Wl,--enable-new-dtags tests/CMakeFiles/collection_extended.dir/Unity/unity_1_cxx.cxx.o tests/CMakeFiles/collection_extended.dir/Unity/unity_0_cxx.cxx.o -o tests/collection_extended  -Wl,-rpath,/usr/local/lib  lib/brotli/libbrotlicommon-static.a  lib/brotli/libbrotlienc-static.a  lib/brotli/libbrotlidec-static.a  lib/libfort/lib/libfort.a  lib/libgtest.a  src/libvt-release.a  -ldl  /usr/lib/x86_64-linux-gnu/libz.so  lib/fmt/libfmt.a  /build/checkpoint/install/lib/libcheckpoint.a  -lpthread  lib/brotli/libbrotlienc-static.a  lib/brotli/libbrotlidec-static.a  lib/brotli/libbrotlicommon-static.a  -lm  lib/libfort/lib/libfort.a  /usr/local/lib/libmpicxx.so  /usr/local/lib/libmpi.so && :
collect2: error: ld returned 1 exit status

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[cz4rs]

Looks good to me!