owp$overplot depends on owp$params which depends on owp$summary

[briandk]

Since, ideally, owp$overplot is just augmented summary information, we end up storing summary information PLUS one column of overplotting information in a separate part of owp:

 owp                       <- AdaptVariablesFromGranovaComputations()
 owp$summary               <- GetSummary(owp)
 owp$group.mean.line       <- GetGroupMeanLine(owp)
 owp$params                <- GetGraphicalParameters(owp)
 owp$overplot              <- AddOverplotInformation(owp$summary, "contrast", 2*owp$params$horizontal.percent)

We should think about how to reduce this dependency chain. But, right now params depend on summary, and overplot depends on both.

(As of 1dc5893)