multi_usrp: Add get_tree() API call

This allows access to the underlying property tree without having to
refer to the device object. Useful for RFNoC objects, where the device
object is not accessible.

host/include/uhd/usrp/multi_usrp.hpp

@@ -127,6 +127,10 @@ public:
      */
     virtual device::sptr get_device(void) = 0;
 
+    /*! Return a reference to the property tree
+     */
+    virtual uhd::property_tree::sptr get_tree(void) const = 0;
+
     //! Convenience method to get a RX streamer. See also uhd::device::get_rx_stream().
     virtual rx_streamer::sptr get_rx_stream(const stream_args_t& args) = 0;
 

host/lib/usrp/multi_usrp.cpp

@@ -405,6 +405,11 @@ public:
         return _dev;
     }
 
+    uhd::property_tree::sptr get_tree() const
+    {
+        return _tree;
+    }
+
     dict<std::string, std::string> get_usrp_rx_info(size_t chan){
         mboard_chan_pair mcp = rx_chan_to_mcp(chan);
         dict<std::string, std::string> usrp_info;

host/lib/usrp/multi_usrp_rfnoc.cpp

@@ -161,6 +161,11 @@ public:
         return nullptr;
     }
 
+    uhd::property_tree::sptr get_tree() const
+    {
+        return _tree;
+    }
+
     rx_streamer::sptr get_rx_stream(const stream_args_t& args_)
     {
         std::lock_guard<std::recursive_mutex> l(_graph_mutex);