Introduce filter_by argument in calculate_variable

[benjello]

Context: calibration with filtering.

It can be a good idea to be able to define a filter_by argument when computing a variable from survey_scenario.

[benjello]

@claireleroy : ce que tu as fait couvrait-il aussi cet aspect ?

[claireleroy]

@benjello Nope only functions by functions (compute_agreggate, create_data_frame_by_entity, compute_pivot_table)...

[benjello]

Should also considering including a calculate_expression method.
And then refactor create_data_frame_by_entity.
But before that we need basic tests for create_data_frame_by_entity with filter_by !