Calculates the New Dale-Chall Readability Formula. In contrast to readability,
which by default calculates all possible indices,
this function will only calculate the index value.
If parameters="PSK", the parameters by Powers-Sumner-Kearl (1958) are used, and if
parameters="old", the original parameters by Dale-Chall (1948), respectively.
This formula doesn't need syllable count.