Starts a new row group of nrows rows on writer -- see parquet_write_column_chunk
below. Every column already known to writer (whether schema-declared or already
chunk-written for an earlier row group) must then receive exactly one
parquet_write_column_chunk call for this row group (or already be a whole column from an
earlier parquet_write_column call) before parquet_finish_row_group. Error stops if a row
group is already open (i.e. called again without an intervening parquet_finish_row_group)
rather than silently abandoning the still-open one.