Issue writing the dataset as parquet in add_features

[radkoff]

I'm running the following (locally, not docker) with the latest from mainline: python -m deepform.data.add_features data/3_year_manifest.csv

And I'm getting this traceback:

Traceback (most recent call last):
  File "/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/Users/evan/deepform/deepform/data/add_features.py", line 263, in <module>
    extend_and_write_docs(
  File "/Users/evan/deepform/deepform/data/add_features.py", line 98, in extend_and_write_docs
    doc_index.to_parquet(pq_index)
  File "/Users/evan/Library/Caches/pypoetry/virtualenvs/deepform-kruIrF0o-py3.8/lib/python3.8/site-packages/pandas/util/_decorators.py", line 199, in wrapper
    return func(*args, **kwargs)
  File "/Users/evan/Library/Caches/pypoetry/virtualenvs/deepform-kruIrF0o-py3.8/lib/python3.8/site-packages/pandas/core/frame.py", line 2365, in to_parquet
    to_parquet(
  File "/Users/evan/Library/Caches/pypoetry/virtualenvs/deepform-kruIrF0o-py3.8/lib/python3.8/site-packages/pandas/io/parquet.py", line 270, in to_parquet
    return impl.write(
  File "/Users/evan/Library/Caches/pypoetry/virtualenvs/deepform-kruIrF0o-py3.8/lib/python3.8/site-packages/pandas/io/parquet.py", line 101, in write
    table = self.api.Table.from_pandas(df, **from_pandas_kwargs)
  File "pyarrow/table.pxi", line 1376, in pyarrow.lib.Table.from_pandas
  File "/Users/evan/Library/Caches/pypoetry/virtualenvs/deepform-kruIrF0o-py3.8/lib/python3.8/site-packages/pyarrow/pandas_compat.py", line 593, in dataframe_to_arrays
    arrays[i] = maybe_fut.result()
  File "/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/concurrent/futures/_base.py", line 432, in result
    return self.__get_result()
  File "/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/concurrent/futures/_base.py", line 388, in __get_result
    raise self._exception
  File "/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/Users/evan/Library/Caches/pypoetry/virtualenvs/deepform-kruIrF0o-py3.8/lib/python3.8/site-packages/pyarrow/pandas_compat.py", line 565, in convert_column
    raise e
  File "/Users/evan/Library/Caches/pypoetry/virtualenvs/deepform-kruIrF0o-py3.8/lib/python3.8/site-packages/pyarrow/pandas_compat.py", line 559, in convert_column
    result = pa.array(col, type=type_, from_pandas=True, safe=safe)
  File "pyarrow/array.pxi", line 265, in pyarrow.lib.array
  File "pyarrow/array.pxi", line 80, in pyarrow.lib._ndarray_to_array
  File "pyarrow/error.pxi", line 84, in pyarrow.lib.check_status
pyarrow.lib.ArrowInvalid: ('Could not convert 0.0 with type str: tried to convert to double', 'Conversion failed for column gross_amount with type object')

I inspected the DataFrame, and the issue appears to be that the document with slug 499480-cancel-68803-13518579030793-_-pdf has 0.0 for gross_amount, which prevents conversion to double.

One solution might be to do:
doc_index['gross_amount'] = doc_index.gross_amount.apply(pd.to_numeric, errors='coerce')
before exporting to parquet format, but I wanted to confirm with you all that this field is supposed to be float, and that the 0.0 amount isn't a mistake. I'm also not sure why no one else has run into this, so maybe something else is up.

[radkoff]

I think this is happening because of mixed dtypes in dollar_amount (both str and float) which doesn't vibe with pyarrow (or at least version 1.0.1 for me). I got the pd.to_numeric solution working, but it required too many other changes around the codebase, so instead I'm casting to str in my PR