Selectively add calculations to better support irregular layouts

[cbailiss]

At the moment, the following line cannot be directly called from outside the pivot table.
private$p_calculationsSet <- TRUE
This makes it impossible to selectively add calculations when building an irregular layout - since pt$generateCellStructure() will always go an re-add the calculations.
Would be handy to be able to specify "asDescendentsOf" or similar, to add at the leaf level of one group (or can this be done simply by calling grp$addDataGroups() on the group and not specifying a level? Is something similar possible for calculations?