Save dataframe into a separate file

[bhou]

Currently, the dataframe (only 5 rows) are stored with the steps in the ananas.yml file. Sometimes, the dataframe is large (100+ columns), which makes the ananas.yml file hard to read.

One solution could be storing the dataframe into a different file. As it is a dedicated data file, we can store more than 5 rows of data for each step.