[minor] diff ex resultset file headers don't account for subsetting when reporting batch confounds

[ppavlidis]

"!!! Warning, this dataset has a batch confound with the factors analysed"

Seen in resultset file (but not the analysis.results.txt file) despite the confound not being relevant in the subset analysis.

[ppavlidis]

https://github.com/PavlidisLab/Gemma/issues/167 is relevant to this. ExpressionExperimentService.getBatchConfound now checks subsets as well. However, the result is still a String, so some more work is needed to address this issue and give a more precise message in the resultset files.

One solution might be to modify ExpressionExperimentService.getBatchConfound to accept subsets as well.