Added infrastructure for json.hpp as a third party library

thirdparty/CMakeLists.txt

 add_subdirectory(KWSys)
+add_subdirectory(NLohmannJson)
 
 if(BUILD_TESTING)
   add_subdirectory(GTest)

thirdparty/NLohmannJson/CMakeLists.txt

+add_library(NLohmannJson INTERFACE)
+target_include_directories(NLohmannJson
+  INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}/json/src
+)

thirdparty/NLohmannJson/Readme.txt

+This is a copy of the upstream JSON For Modern C++ library located at
+https://github.com/nlohmann/json. Do not make changes directly to this repo
+but instead to the upstream repository.  Update this copy of json.hpp by running
+the update.sh script.

thirdparty/NLohmannJson/update.sh

+#!/usr/bin/env bash
+
+set -e
+set -x
+shopt -s dotglob
+
+readonly name="NLohmannJson"
+readonly ownership="JSON For Modern C++ Upstream <robot@adios2>"
+readonly subtree="thirdparty/NLohmannJson/json"
+readonly repo="https://github.com/nlohmann/json.git"
+readonly tag="v2.1.1"
+readonly shortlog="true"
+readonly paths="
+LICENSE.MIT
+src/json.hpp
+"
+
+extract_source () {
+    git_archive
+}
+
+. "${BASH_SOURCE%/*}/../update-common.sh"