[BUG] Doesn't take into account proper time difference when cells are in colspan

[dipamsen]

https://github.com/metakgp/gyft/blob/705462d36f3ea03d878da0e1886258fbb45eed0a/timetable/extractor.py#L106-L109

In this code, it uses timings[index] as start time of the course, where index is the index of the cell in the row. However, this is not correct when the ERP timetable contains colspans in earlier cells, because that has to be taken into account (timings[index + no of skipped cells due to colspan])

This causes issue in the generated time table: