New: preparedViewAtColumn:row:
These methods allow to enumerate visible data views or data views in specified columns and rows.
-enumerateAvailableViewsUsingBlock: is the counterpart of cocoa's -enumerateAvailableRowViewsUsingBlock: except that it enumerates data views instead of CPTableRowView and the block has an additional column parameter.
This commit reverses the way views are stored and accessed in the table view: rows>columns instead of columns>rows.
Applied the second method where it is relevant, making the code more compact and readable.