Add swap command

Per-channel current swapping, for use with Zotino in the meantime on
HWRevs < 3.0 as the Zotino headers are swapped on the Thermostat.

@@ -414,1 +414,4 @@
 
+    fn swap_tec_polarity (socket: &mut TcpSocket, channels: &mut Channels, channel: Option<usize>) -> Result<Handler, Error> {
+        for c in 0..CHANNELS {
+            if channel.is_none() || channel == Some(c) {

@@ -122,0 +122,4 @@
+            -i_set
+        } else {
+            i_set
+        }

@@ -415,0 +415,4 @@
+    fn swap_tec_polarity (socket: &mut TcpSocket, channels: &mut Channels, channel: Option<usize>) -> Result<Handler, Error> {
+        for c in 0..CHANNELS {
+            if channel.is_none() || channel == Some(c) {
+                channels.channel_state(c).swap_tec_polarity = !channels.channel_state(c).swap_tec_polarity;