ToConfig with columns order for CSV

[pgrandinetti]

• Added new ToConfig to force order of columns in CSV.
• Added tests for new ToConfig.

[pgrandinetti]

Hi. I know QFrame doesn't hold order of columns, but I needed it in the CSV. Didn't find a way to do it, so I implemented it.
Feel free to close if out of scope, and feedback's welcome.
Thanks for your work.

➜  qframe git:(columns_order_tocsv) go test
PASS
ok  	github.com/tobgu/qframe	0.020s

[tobgu]

Great, thanks!

Seems functionally correct. There are a few nitpicks on style that you may want to consider.

[tobgu]

Just some nitpicking and suggestions for how to make the code a bit shorter and more idiomatic:

		iterCols = make([]namedColumn, len(qf.columns))    // Pre-set length
		for i := range conf.Columns {    // Use range expression
			cName := conf.Columns[i]  // camelCase name
			if col, ok := qf.columnsByName[cName]; !ok {
				return qerrors.New("ToCSV", fmt.Sprintf("%s: column does not exist in QFrame", cName))
			}
                        // Skip the else-clause since there is a return in the if-clause.

                        // Set element using indexing rather than append (since we now have a length set)
			iterCols[i] = col
		}

[pgrandinetti]

Nice, thank you @tobgu
Made all recommended changes, except the last one: when col is defined inline in the if, then it needs a else branch to be used, or the compiler will reject the code. See minimal example here: https://go.dev/play/p/2bowv-oHDl5

[tobgu]

LGTM!