Fix Visual Studio 2017 warnings

Fix warning C4706 on Visual Studio 2017 - fixes #784

Set GENERATE_TAGFILE in Doxyfile

Lots of tests have this warning.

Also moved out of for loop, doesn't need to be done every loop.

This causes test-udt to crash due to bad iterator

Allows documentation to be linked from other projects to https://nlohmann.github.io/json using Doxygen with the tag:
TAGFILES = $(SOME_PATH)/nlohmann_json.tag=https://nlohmann.github.io/json

using more CMake

Add info for the vcpkg package.