Correct issues with Symm algo and UI

Refs #10671
Unit test the dodgy functionality

Refs #10671
Make Symmetrise docs more generic

Refs #10671
Correct spelling mistake

Refs #10671
Clarify what the data is made symmetrical about

Refs #10671
Merge remote-tracking branch 'origin/bugfix/10671_symmetrise_ui_and_algo_bugs'