add better interface handling

src/dac.rs

@@ -131,4 +131,8 @@ impl<'a, T: DacInterface> DpcmDac<'a, T> {
             false
         }
     }
+
+    pub fn disable_output(&mut self) {
+        self.output.disable();
+    }
 }

src/interface.rs

@@ -1,4 +1,5 @@
 pub trait DacInterface {
     // write the amplitude (0->100%)
+    fn disable(&mut self);
     fn write_amplitude(&mut self, amplitude: u8);
 }