PARQUET-1040: Add missing writer methods

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

Closes #365 from xhochy/PARQUET-1040 and squashes the following commits:

ef359ef [Korn, Uwe] PARQUET-1040: Add missing writer methods
diff --git a/src/parquet/file/writer.cc b/src/parquet/file/writer.cc
index 17afa02..cafa6b8 100644
--- a/src/parquet/file/writer.cc
+++ b/src/parquet/file/writer.cc
@@ -45,6 +45,15 @@
   return contents_->current_column();
 }
 
+int RowGroupWriter::num_columns() const {
+  return contents_->num_columns();
+}
+
+int64_t RowGroupWriter::num_rows() const {
+  return contents_->num_rows();
+}
+
+
 // ----------------------------------------------------------------------
 // ParquetFileWriter public API
 
@@ -84,6 +93,18 @@
   return contents_->schema()->Column(i);
 }
 
+int ParquetFileWriter::num_columns() const {
+  return contents_->num_columns();
+}
+
+int64_t ParquetFileWriter::num_rows() const {
+  return contents_->num_rows();
+}
+
+int ParquetFileWriter::num_row_groups() const {
+  return contents_->num_row_groups();
+}
+
 const std::shared_ptr<const KeyValueMetadata>& ParquetFileWriter::key_value_metadata()
     const {
   return contents_->key_value_metadata();