Increased memory usage in make multi-run

[WenzDaniel]

Describe the bug
Currently there is a larger memory use by

 st.make(run_ids, data_type)

compared to

for run in run_ids:
     st.make(run, data_type)

The problem arises from the fact that the result in the worker processes might not get garbage collected. The issue is described also in https://stackoverflow.com/questions/54108434/garbage-collecting-python-subprocesses, but currently there is not much we can do about it.

[WenzDaniel]

Okay issue seems to be a different one although I cannot pin point it at the moment

[JoranAngevaare]

Fixed in #581?