condMean

<code>numeric</code> vector to be averaged

(list of) <code>factor(s)</code> for which the mean will be calculated

index

<code>logical</code> If <code>TRUE</code> <code>NA</code> values will be removed

<code>condMean</code> works like a very fast version of tapply with <code>FUN=mean</code>.

Extends the out of memory vectors of 'ff' with
statistical functions and other utilities to ease their usage.