fillNA: Fill large spaces in profiles with NA
In Starr: Simple tiling array analysis of Affymetrix ChIP-chip data

This function fills spaces in the profiles, that exceed a given distance. It is called by getProfiles, if method ”middle” is chosen.

1	fillNA(distribution, mapping, upstream, downstream, gffAnno, distance, spacing)

`distribution`	a list containing the profiles for each feature
`mapping`	a mapping of, which was constructed by the mapFeatures function
`gffAnno`	a data frame containing the annotation of the features of interest
`upstream`	how many basepairs upstream of the feature start (feature start on the crick strand is end in gffAnno) should be taken.
`downstream`	how many basepairs downtream of the feature start (feature end on the crick strand is start in gffAnno) should be taken.
`distance`	spaces, exceeding this distance are filled with NA
`spacing`	probe spacing on the chip