'sort' step to Python postprocess

[vdestraitt]

Ex 1: sort on one column

This 'sort' step config in our VQB "language"...

{
   name: 'sort',
   columns: ["foo"]
   order: ["asc"] # 'asc' by default
}

... should yield the following postprocess pipeline:

{
   sort:
     columns: ["foo"]
     order: ["asc"]
}

Ex 2: sort on multiple columns

This 'sort' step config in our VQB "language"...

{
   name: 'sort',
   columns: ["foo", "bar"]
   order: ["asc", "desc"]
}

... should yield the following postprocess pipeline:

{
   sort:
     columns: ["foo", "bar"]
     order: ["asc", "desc"]
}