Standard functions such as ds.summary should cope with dates #451

At the moment, ds.summary can only deal with data that is numeric, integer or character. For a vector of dates it throws an error.

A general note on ds.summary is that it should probably rearchitected to be a generic function rather than having a load of if(class==numeric){do this} if(class==char){do that} statements in it. And that is probably relevant for ds.mean too?

It might be useful for users to enable data arithmetic (e.g. difference between 2 dates in days).