Fix/dt 483 add previous file

[pirmink]

To de-accumulate precipitation, one needs the precipitation of the step before.

[pirmink]

The file of the step before is stored in the variable previous_file, but was not passed into neither load_grib() nor preprocess(). @ninaburg suspects that the precipitation for step t is de-accumulated with step t = 0 instead of step t - 1 and suggested this change. The simple addition of previous_file however leads to pytest failing. I therefore mark this PR as draft. @victoria-cherkas please check if the change is correct and how the tests need to be adapted to pass pytest.

[victoria-cherkas]

Thanks for spotting this! I had to add one smalll change to fix the tests