loadtxt - add `usecols` option

[rzehumat]

Motivation

Would it be possible to add an option to skip the loading of certain columns when using the loadtxt from subroutine from stdlib_io?

The use case could be for loading tabular files with missing data. For example, loading a file with the table below would with fmt="*" would fail (unless skiprows=4). However, sometimes one would need just to load columns 1-3.

1.00E+00  2.00E+00  3.00E+00  5.00E+00
4.00E+00  5.00E+00  6.00E+00  5.00E+00
7.00E+00  8.00E+00  9.00E+00  5.00E+00
7.00E+00  8.00E+00  9.00E+00  
7.00E+00  8.00E+00  9.00E+00  5.00E+00
7.00E+00  8.00E+00  9.00E+00  5.00E+00

Prior Art

The option would be similar to usecols option in numpy

Additional Information

Other options to treat missing values are possible -- e.g. supplying NaN to missing values or skipping rows or columns with missing data.

[ivan-pi]

How would you count columns, if there was a gap?

1.00E+00  2.00E+00  3.00E+00  5.00E+00 3.00E+00
4.00E+00  5.00E+00  6.00E+00  5.00E+00 3.00E+00
7.00E+00  8.00E+00  9.00E+00  5.00E+00 3.00E+00
7.00E+00  8.00E+00  9.00E+00           3.00E+00
7.00E+00  8.00E+00  9.00E+00  5.00E+00 3.00E+00
7.00E+00  8.00E+00  9.00E+00  5.00E+00 3.00E+00

Is the value 3.00E+00 on line 4 part of column 4 or column 5?