[FEAT] add t0 to FeatureCollection.calculate()

[jonasvdd]

Suppose you want to (1) extract features for multiple groups (e.g. multiple users or machines), and (2) your data is irregularly sampled (or gaps occur), and (3) you want to to ensure that the start times of the features over the multiple groups are the same.

This can be achieved by

• segmenting manually using segment_start_idxs and segment_end_ids ; but then you:
  • lose the window stride information in the column
  • you must create a segment range for each window-stride configuration; which is labor-intensive

This can all be solved by providing a t0 to the calculate method;

• t0 is the first timestamp which will be used to create the window-stride segments. All groups will have the same indices.