Add smarter typecasting to the samplesheet converter

[nvnieuwk]

Smarter typecasting should be added to the samplesheet converter to keep the conversion consistent across similar schemas. e.g.:

{
  "type": "string",
  "oneOf": [
    { "format": "file-path" },
    { "const": "hi" }
  ]
}

and

{
  "oneOf": [
    { 
      "type": "string",
      "format": "file-path"
    },
    {
      "type": "string",
      "const": "hi"
    }
  ]
}

Are functionaly the same schemas but they have different results:

1. Does not convert the file into a file object
2. Does convert the file into a file object