'percentage' step to Python postprocess

[vdestraitt]

Exemple 1: with groups and result in new column

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

{
   name: 'percentage'
   new_column: 'new_col'
   column: 'bar'
   group: ['foo']
}

... should yield the following postprocess pipeline:

{
   percentage:
     new_column: 'new_col'
     column: 'bar'
     group_cols: ['foo']
}
{
   formula:
     new_column: 'new_col' # writes result inplace
     formula: "new_col / 100" # We need to divide by 100 to get a result between 0 and 1 while the percentage step returns values between 0 and 100
}

Exemple 2: without groups and result in existing column

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

{
   name: 'percentage'
   column: 'bar'
}

... should yield the following postprocess pipeline:

{
   percentage:
     column: 'bar'
}
{
   formula:
     new_column: 'bar' # writes result inplace
     formula: "bar / 100" # We need to divide by 100 to get a result between 0 and 1 while the percentage step returns values between 0 and 100
}