PARQUET-605: Expose schema node in ColumnDescriptor

Author: Korn, Uwe <Uwe.Korn@blue-yonder.com>

Closes #99 from xhochy/parquet-605 and squashes the following commits:

0d444e5 [Korn, Uwe] PARQUET-605: Expose schema node in ColumnDescriptor

Author: xhochy

src/parquet/schema/descriptor.h

@@ -53,6 +53,8 @@ class ColumnDescriptor {
 
   const std::shared_ptr<schema::ColumnPath> path() const;
 
+  const schema::NodePtr& schema_node() const { return node_; }
+
   int type_length() const;
 
   int type_precision() const;