implemented single marker analysis and multimarker analysis on means.
The two method only implement on phenotypic data of means and does not
connect to my class design using PhenotypicData, SingleSiteAnalysis and
MultiSiteAnalysis class. It only use GenotypicData object after
restricted. 
And the nest step is to implement my original design using S3 class
method to support PhenotypicData, SingleSiteAnalysis and
MultiSiteAnalysis.