Invalid concatenation for multiple parquet files with different columns structure 

If users specify multiple parquet files which has different column structure, the outcome is concat with column direction (not row direction).

```bash
$ poetry run parquet-tools csv ./tests/test1.parquet ./tests/test0.parquet
one,two,three,a,b,c,d
-1.0,foo,True,,,,
,bar,False,,,,
2.5,baz,True,,,,
```