Add Niels Lohmann's JSON for Modern C++ header-only library

[hqucms]

Backport of #5641 for 10_6_X.

Add an external and a tool file for Niels Lohmann's JSON for Modern C++
header-only library, from https://github.com/nlohmann/json .

Can be used in CMSSW by adding to a BuildFile.xml:

and to a source or header file:
#include <nlohmann/json.hpp>
using json = nlohmann::json;

or, to include only the forward-declation:
#include <nlohmann/json_fwd.hpp>
using json = nlohmann::json;

See https://github.com/nlohmann/json/blob/develop/README.md for some
documentation.

[cmsbuild]

A new Pull Request was created by @hqucms (Huilin Qu) for branch IB/CMSSW_10_6_X/gcc700.

@cmsbuild, @smuzaffar, @mrodozov, @tulamor can you please review it and eventually sign? Thanks.
cms-bot commands are listed here

[smuzaffar]

please test

[cmsbuild]

The tests are being triggered in jenkins.

• CMSSW_10_6_X_2020-08-03-2300/slc7_amd64_gcc700: https://cmssdt.cern.ch/jenkins/job/ib-run-pr-tests/8559/console Started: 2020/08/04 17:08

[cmsbuild]

+1
Tested at: e8c6f3c
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-34ee44/8559/summary.html
CMSSW: CMSSW_10_6_X_2020-08-03-2300
SCRAM_ARCH: slc7_amd64_gcc700

[cmsbuild]

Comparison job queued.

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-34ee44/8559/summary.html

Comparison Summary:

• No significant changes to the logs found
• Reco comparison results: 4 differences found in the comparisons
• DQMHistoTests: Total files compared: 34
• DQMHistoTests: Total histograms compared: 3214618
• DQMHistoTests: Total failures: 2
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 3214282
• DQMHistoTests: Total skipped: 334
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 33 files compared)
• Checked 140 log files, 14 edm output root files, 34 DQM output files

[smuzaffar]

+externals

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next IB/CMSSW_10_6_X/gcc700 IBs (tests are also fine). This pull request will now be reviewed by the release team before it's merged. @silviodonato, @dpiparo, @qliphy (and backports should be raised in the release meeting by the corresponding L2)