made pandas dataframe optional to prevent serialization

[trbKnl]

Solves

d3i-infra#23

Solution

Make data frames optional so no serialization has to take place. Current code remains working.

The change is small, but the effects on performance are large in case the dataframes become extremely large.
This sometimes an issue for tiktok, when people have watch history going into the million.

To test

In script.py use for example:

data_frame = { 
    "variable_x": {"0":1, "1":3},
    "variable_y": {"0": "appel", "1": "peer"},
}