New extrusion class: ExtrusionMultiPath

This is similar to an ExtrusionLoop, but it is open.
It may contain multiple chained paths with differing parameters.
This allows one to have a hierarchy of paths, where the ExtrusionEntityCollection
will be chained by the G-code generator, but ExtrusionMultiPath will not.

xs/xsp/typemap.xspt

@@ -78,6 +78,9 @@
 %typemap{ExtrusionEntityCollection*};
 %typemap{Ref<ExtrusionEntityCollection>}{simple};
 %typemap{Clone<ExtrusionEntityCollection>}{simple};
+%typemap{ExtrusionMultiPath*};
+%typemap{Ref<ExtrusionMultiPath>}{simple};
+%typemap{Clone<ExtrusionMultiPath>}{simple};
 %typemap{ExtrusionPath*};
 %typemap{Ref<ExtrusionPath>}{simple};
 %typemap{Clone<ExtrusionPath>}{simple};