More intuitive widget for array of numbers

[sverhoeven]

The form renders a unique array of numbers as a select tag with the multiple attrib. Having to use ctrl/shift to select multiple items is something the user needs to know.

You could use checkboxes by adding

    prop1:
      items:
        hisd:
          "ui:widget": "checkboxes"`

to ui schema, but this only works for small molecules.

Another option would be to use https://react-select.com/ .

Spin off from #62 (review) comment.