Add AudioSynthWaveformDc read()

gui/index.html

 		be specified, but periods longer than 1 second may be automatically
 		shortened for small level changes, due to numerical precision limits.
 	</p>
+	<p class=func><span class=keyword>read</span>();</p>
+	<p class=desc>Read the current level.  Returns -1.0 to 1.0.  This can be
+		useful for monitoring the amplitude after configuring a slow change.
+	</p>
 	<!--
 <h3>Examples</h3>
 	<p class=exam>File &gt; Examples &gt; Audio &gt;

synth_dc.h

 		state = 1;
 		__enable_irq();
 	}
+	float read(void) {
+		int32_t m = magnitude;
+		return (float)m * (1.0 / 2147418112.0);
+	}
 	virtual void update(void);
 private:
 	uint8_t  state;     // 0=steady output, 1=transitioning