Add accessor to ParquetWriter to get current data size

[asfimport]

Reporter: Ryan Blue / @rdblue
Assignee: Ryan Blue / @rdblue

Related issues:

• ParquetWriter.getDataSize NullPointerException after closed (is related to)

PRs and other links:

• PR #212

_{Note: This issue was originally created as PARQUET-308. Please see the migration documentation for further details.}

[asfimport]

Ryan Blue / @rdblue:
Merged #212.