expressionByFeature: Getting expression value by feature from an ExpressionSet
In Starr: Simple tiling array analysis of Affymetrix ChIP-chip data

This function gets the expression of a specified feature (e.g. orf, gene) from an ExpressionSet.

1	expressionByFeature(eSet, fkt, method="median")

`eSet`	An ExpressionSet, containing the normalized expression values
`fkt`	Function to convert the featureNames (e.g. affy IDs) of eSet to the required features (e.g. ORFs)
`method`	If one feature (e.g. ORF) has more than one feature (e.g. affy ID) on the chip, the mean/median over the intensities is taken