Add 'scaled column addition' command.

Add 'scaled column addition' command.

Example usage: removing linear drifts with

  scaled_column_addition --block retract
    --input_column_1 'surface deflection (m)'
    --input_column_2 'surface distance (m)'
    --output_column 'flattened surface deflection (m)
    --scale_1 1 --scale_2 -1
    --scale_2_name 'surface deflection offset|non-contact slope'

Also:
* Allow `None` as a column name in ColumnAccessCommand._get_column().
* Add 'force zero non-contact slope' option to SurfaceContactCommand.
* Convert slopes from surface contact fit from deflection/point to
  deflection/distance.
* Fix the returned deflection offset so it is a float and not a
  length-one tuple (by removing the trailing comma).