PPrecision should create a string, not a float, in the JSON output

[DougBurke]

I am hoping this is an error in the Vega-Lite 3.3.0 specification - see vega/vega-lite#5190 for a discussion - but PPrecision should be generating a string rather than a float.

(I've finally integrated your test suite into hvega, where I generate the JSON for each test and then compare it against the checked-in version to ensure the output hasn't changed; it means I have a bunch of JSON files I can easily validate against the Vega Lite schema, which finds the odd strange case like this)

[jwoLondon]

Thanks for raising this over on Vega-Lite. Looks like it will be solved in the next VL release vega/vega-lite#5191 and the elm-vegalite and hvega specs can remain as is.