[TDF] Template TInterface over the derived TDataSource type

This will allow, in the future, to optimize loading of entries
from TDataSources: `GetColumnReaders` does not have to pass through
virtual calls to `TDataSource` anymore, which means that concrete
TDS's can provide custom, possibly lazy column readers which
TInterface will use directly.

Author: eguiraud

tree/treeplayer/inc/ROOT/TDFInterface.hxx

@@ -97,15 +97,15 @@ namespace TTraits = ROOT::TypeTraits;
 * \brief The public interface to the TDataFrame federation of classes
 * \tparam T One of the "node" base types (e.g. TLoopManager, TFilterBase). The user never specifies this type manually.
 */
-template <typename Proxied>
+template <typename Proxied, typename DataSource = void>
 class TInterface {
    using ColumnNames_t = TDFDetail::ColumnNames_t;
    using TFilterBase = TDFDetail::TFilterBase;
    using TRangeBase = TDFDetail::TRangeBase;
    using TCustomColumnBase = TDFDetail::TCustomColumnBase;
    using TLoopManager = TDFDetail::TLoopManager;
    friend std::string cling::printValue(::ROOT::Experimental::TDataFrame *tdf); // For a nice printing at the prompt
-   template <typename T>
+   template <typename T, typename W>
    friend class TInterface;
 
    const std::shared_ptr<Proxied> fProxiedPtr;     ///< Smart pointer to the graph node encapsulated by this TInterface.