Improving catalog generation

Support multiple primary aircraft in a package, which has several
aircraft developers have requested. This adjusts the handling of
the variant-of and primary-set tags.

Additionally, this adds partial unit-test coverage for -set.xml
scanning and catalog XML generation. 

Run ./test_catalog to run unit-tests of the catalog generation.

catalog/testData/OtherDir/settings-common.xml

@@ -0,0 +1,12 @@
+<?xml version='1.0' encoding='UTF-8'?>
+
+<!-- this file exists to test including + overlaying in the set XML -->
+
+<PropertyList>
+    <views>
+      <view n="0">
+        <foo>dsdddd</foo>
+      </view>
+    </views>
+
+</PropertyList>