Add an option for the aggregation "concatenate" to only concatenate distinct values

[Seddryck]

At the moment if the field that must be concatenated contains the same value in different rows, this value is displayed multiple times in the concatenation.

Expl:

Supplier	Fruit	Qty
Foo	Apple	50
Foo	Apple	10
Foo	Banana	20
Bar	Apple	30

With the alteration

<summarize>
   <concatenation column="Fruit" type="text" separator=", "/>
   <sum column="Qty" type="numeric" />
   <group-by>
      <column identifier="Supplier">
   </group-by>
<summarize>

Supplier	Fruit	Qty
Foo	Apple, Apple, Banana	80
Bar	Apple	30

But with the new attribute distinct set to true of this aggregation, we're able to only have once the fruit apple for supplier Foo

<summarize>
   <concatenation column="Fruit" type="text" separator=", " distinct="true"/>
   <sum column="Qty" type="numeric" />
   <group-by>
      <column identifier="Supplier">
   </group-by>
<summarize>

Supplier	Fruit	Qty
Foo	Apple, Banana	80
Bar	Apple	30