Fix DAX engine behaviour for union of more than 2 tables.

[debruyck]

The DAX union statement behaves weird when applied to more than 2 tables.  Below 5 tables have the same list of columns in the same order, and still union doesn't behave as expected.  Example code below for the work-around which is a bunch of SELECTCOLUMNS statements.

1. this works: UNION(f_aux1_ETR_IT_act, f_aux1_ETR_IT_etc) works as expected.  But for 3 tables it doesn't.   
2. nesting unions only taking union of 2 tables at a time, didn't work either 

3. workaround is below, using explicit selectcolumns in the union.   Cumbersome

f_act_etc = -- UNION mixes columns when using more than 2 tables, also when nesting UNIONs, only work-around was this

    UNION(

        SELECTCOLUMNS(f_aux1_ETR_IT_act,

            "amount" ,[amount],

            "amount (md)", [amount (md)],

            "amount type", [amount type],

            "Budget Code", [Budget Code],

            "IT_Concepts_Lvl3a_NEW", [IT_Concepts_Lvl3a_NEW],

            "OBS_FEATURE", [OBS_FEATURE],

            "OBS_RESOURCE_NAME", [OBS_RESOURCE_NAME],

            "OBS_RESOURCE_TYPE", [OBS_RESOURCE_TYPE],

            "po_number", [po_number],

            "supplier_id", [supplier_id],

            "value_date", [value_date]

            ),

        SELECTCOLUMNS(f_aux1_ETR_IT_etc,

            "amount", [amount],

            "amount (md)", [amount (md)],

            "amount type", [amount type],

            "Budget Code", [Budget Code],

            "IT_Concepts_Lvl3a_NEW", [IT_Concepts_Lvl3a_NEW],

            "OBS_FEATURE", [OBS_FEATURE],

            "OBS_RESOURCE_NAME", [OBS_RESOURCE_NAME],

            "OBS_RESOURCE_TYPE", [OBS_RESOURCE_TYPE],

            "po_number", [po_number],

            "supplier_id", [supplier_id],

            "value_date", [value_date]

        ),

        SELECTCOLUMNS(f_aux2_t_fcst_act,

            "amount", [amount],

            "amount (md)", [amount (md)],

            "amount type", [amount type],

            "Budget Code", [Budget Code],

            "IT_Concepts_Lvl3a_NEW", [IT_Concepts_Lvl3a_NEW],

            "OBS_FEATURE", [OBS_FEATURE],

            "OBS_RESOURCE_NAME", [OBS_RESOURCE_NAME],

            "OBS_RESOURCE_TYPE", [OBS_RESOURCE_TYPE],

            "po_number", [po_number],

            "supplier_id", [supplier_id],

            "value_date", [value_date]

        ),

        SELECTCOLUMNS(f_aux2_t_fcst_etc,

            "amount", [amount],

            "amount (md)", [amount (md)],

            "amount type", [amount type],

            "Budget Code", [Budget Code],

            "IT_Concepts_Lvl3a_NEW", [IT_Concepts_Lvl3a_NEW],

            "OBS_FEATURE", [OBS_FEATURE],

            "OBS_RESOURCE_NAME", [OBS_RESOURCE_NAME],

            "OBS_RESOURCE_TYPE", [OBS_RESOURCE_TYPE],

            "po_number", [po_number],

            "supplier_id", [supplier_id],

            "value_date", [value_date]

        ),

        SELECTCOLUMNS(f_aux3_ETC_softw_etc,

            "amount", [amount],

            "amount (md)", [amount (md)],

            "amount type", [amount type],

            "Budget Code", [Budget Code],

            "IT_Concepts_Lvl3a_NEW", [IT_Concepts_Lvl3a_NEW],

            "OBS_FEATURE", [OBS_FEATURE],

            "OBS_RESOURCE_NAME", [OBS_RESOURCE_NAME],

            "OBS_RESOURCE_TYPE", [OBS_RESOURCE_TYPE],

            "po_number", [po_number],

            "supplier_id", [supplier_id],

            "value_date", [value_date]

        )

    )