Calculates Guti<U+00E9>rrez de Polini's F<U+00F3>rmula de comprensibilidad (Guti<U+00E9>rrez, 1972,
as cited in Fern<U+00E1>ndez, 2016).
In contrast to readability
,
which by default calculates all possible indices,
this function will only calculate the index value.