Fix/metar reading (#354)

* Account for empty ICAO; added tests

* Fix null returns

* Fix typo in volume units display

* Add version field for bug report template

* Some more changes to the bug report template

tests/data/aviationweather/empty.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<response xmlns:xsd="http://www.w3.org/2001/XMLSchema"
+          xmlns:xsi="http://www.w3.org/2001/XML-Schema-instance" version="1.2"
+          xsi:noNamespaceSchemaLocation="http://aviationweather.gov/adds/schema/metar1_2.xsd">
+    <request_index>26064774</request_index>
+    <data_source name="metars"/>
+    <request type="retrieve"/>
+    <errors>
+        <error>Invalid station string: station string cannot be empty</error>
+    </errors>
+    <warnings/>
+    <time_taken_ms>0</time_taken_ms>
+</response>