Allow array values in json

[adlerfaulkner]

Fixes #32

[ThibaultGerrier]

Ideally the .getData would always return string[] - the list of element where each element should get a new triple.
With your changes here it would now return (string|string[])[]. For some reason this still works and produces the expected result (because the code has a bunch of helper.addArray() and helper.curArray() sprinkled after getting the data)

Because of this it is now possible to create invalid json-ld. For instance:
in the mapping: rml:reference "favorite-numbers.*" ;
in the data: "favorite-numbers": [[ 3, 33, 13 ], 1]
the resulting invalid jsonld: "@value": [ "3", "33", "13" ],
where instead this should default back to the "old" mode and output 2 triples, one with "@value": "3,33,13" and the other with 1

I think the easiest way to fix this would be to make this case of "one element collection with an array" explicit
something like:

    const arr =  JSONPath({
      path: `${this.paths[index]}${splitter}${sel}`,
      json: this.json,
      resultType: selector.startsWith('PATH~') ? 'pointer' : 'value',
    })
      .filter((e) => e !== null && e !== undefined)

    if(arr.length === 1 && Array.isArray(arr[0])) {
      return arr[0].map(e => e.toString())
    }
    return arr.map(e => e.toString())

that way the return type also stays string[]

[adlerfaulkner]

@ThibaultGerrier sounds good. Pushed a change with your suggestion.