Retrieves the current set of weights i.e. the state

## Parameters

- document_name: String - document name
- tokens: List of tokens each being a String

## Example

iex> ContentIndexer.TfIdf.Calculate.tf_idf("test_file.md", ["bread","butter"])
{:ok, [
    {"test_file_1.md", [{"butter", 0}, {"bread", -0.234}]},
  ]
}