Bf/column option

See merge request fsl/ukbparse!99

the two was being loaded. Should be union.

unnamed.

Enh/select by column name

See merge request fsl/ukbparse!98

are no data columns to be exported.

the input was.

are ordered according to their positions in the input files. Original
importing.columnsToLoad code did not guarantee ordering of any kind.

biobank-style column names.

different order to the file column order. Now the columnsToLoad function
returns column lists ordered according to how they appear in the file.

Also (the catalyst to this refactoring) discovered that the pd.read_csv
index_col argument needs to be specified relative to the usecols argument.
Silly that.

This commit is likely to have broken something.