preload json file into the namespace

[sswan]

We need a way to get a dict of variables into the thresh namespace to help us in doing asserts. Maybe something like this:

$ cat vars.json
{
  "density_gold": 123.456
}
$ thresh preload=vars.json A=data.txt cat "errors=density_gold-Adensity" assert "max(errors) < 1.0e-6"

[sswan]

Maybe it would be better like this:

$ thresh foo.txt A=bar.csv variables.json pi_approx=3 assert "density_gold / pi_approx == 41.0"

That might be best. I'll need to refactor how data is read in a represented.