This change fixes a problem in Workbench when setting columns to YErr
Previously this only worked if all Y columns had an associated YErr column. If that
wasn't the case then the associations between Y and YErr cols got mixed up
I have saved the link between the YErr and Y column into the C++ table workspace
object via changes to ITableWorkspace and the Column class
I've also modified the model\view\presenter for the table workspace to set and get
the link via the ITableWorkspace interface
The code in marked_columns.py has been modified to remove the _add function because
the logic required for adding x, y and yerr was sufficiently different that trying
to push them through a common _add function was obscuring the code
Finally I've removed the labelindex from the ErrorColumn object in the python code
because it can be worked out on the fly (and I didn't want to have to save this also
into the C++ table workspace in addition to the linked column index)

As part of the test updates I have removed the test
TableWorkspaceDisplayModelTest.test_initialise_marked_columns_yerr_before_y_doesnt_mark_yerr
because we do require the ability to add a yerr column before the corresponding y column
The model gets refreshed whenever the C++ table workspace is updated (eg when the type of a column
is updated) as part of a replace_workspace call. The model __init__ function builds up the column
set left to right and a user may have set up a Y err column which is to the left of the corresponding
Y column.