Initial commit with telemetry unit label support

Committing initial code that supports the generation of an APRS unit/label
string. This was testing by sending telemetry labels to APRS-IS and
checked on aprs.fi. Not cleaned up yet.

aprslib/packets/__init__.py

@@ -1,2 +1,3 @@
 from aprslib.packets.position import PositionReport
 from aprslib.packets.telemetry import TelemetryReport
+from aprslib.packets.telemetryunitlabels import TelemetryUnitLabelsReport

aprslib/packets/telemetryunitlabels.py

@@ -0,0 +1,43 @@
+from aprslib.packets.base import APRSPacket
+
+class TelemetryUnitLabelsReport(APRSPacket):
+    format = 'raw'
+    telemetrystation = "N0CALL"
+    a1 = "BITS"
+    a2 = "BITS"
+    a3 = "BITS"
+    a4 = "BITS"
+    a5 = "BITS"
+    b1 = "EN"
+    b2 = "EN"
+    b3 = "EN"
+    b4 = "EN"
+    b5 = "EN"
+    b6 = "EN"
+    b7 = "EN"
+    b8 = "EN"
+    comment = ''
+
+    def _serialize_body(self):
+
+        body = [
+            ':{0} :UNIT.'.format(self.telemetrystation),  # packet type
+            self.a1,
+            self.a2,
+            self.a3,
+            self.a4,
+            self.a5,
+            self.b1,
+            self.b2,
+            self.b3,
+            self.b4,
+            self.b5,
+            self.b6,
+            self.b7,
+            self.b8,
+        ]
+        tmpbody = ",".join(body)
+        badcomma = tmpbody.index(",")
+
+        # remove static but erroneous comma between UNIT. and a1 value
+        return tmpbody[:badcomma] + tmpbody[badcomma+1:]