Be more explicit about assocComp()$association meaning

[alexg9010]

When talking about the batch correction (https://github.com/al2na/methylKit/blob/ab60a62e0bd610718fcbb76e9505957c8c3ec15f/vignettes/methylKit.Rmd#L360-L369), we should probably be more explicit about the output of assocComp() (https://github.com/al2na/methylKit/blob/ab60a62e0bd610718fcbb76e9505957c8c3ec15f/R/batchControl.R#L180-L185).

This recent issue (https://github.com/al2na/methylKit/issues/247 ) shows that people might not be aware that the association component contains p-values, thus should not be interpreted as percentages of association or similar.

I suggest renaming the list item to something more explicit, like association_test or association_pval.