Add tests for json_reader function and sample json data

[Bisaloo]

New PR based on @RichardMN's PR #425. Rebased on top of current master for clearer diff. I had to open a new PR as I don't have rights to write directly on Richard's fork.

Fix #425.

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[RichardMN]

New PR based on @RichardMN's PR #425. Rebased on top of current master for clearer diff. I had to open a new PR as I don't have rights to write directly on Richard's fork.

I'm really not sure why you don't have the rights to put changes on my fork since other folks from epiforecasts have been able to in the past when I've submitted PRs... If you can tell me which switch to flip on my forks I'll try doing that.