column commands
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bcsv_column_map <out-handle> <in-handle> <column> <map-fun <ret-field> <field> <row> [xargs]> [xargs]
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bcsv_column_iter <handle> <column> <iter-fun <field> <row> [xargs]> [xargs]
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bcsv_column_remove <handle> <column>
delete column -
bcsv_column_add <handle> <column>
add empty column