Refs #9056. Return the behaviour changed in #8774.

If tables are not cleared before filling, we get problems.
Refs #9056. Move column type setting to fill function.

Otherwise column types are set on creating the table and not on when it
gets updated.
Refs #9056. Minor style improvements.
Merge remote-tracking branch 'origin/feature/9056_mantid_table_replace_all_y_columns'