Fix/days of supply calculation

[ReneSchroederLJ]

Description

• adjusted the calculation of days of supply to be in line with the standard
• notable differences are:
  • actual item stock is displayed on the previous day
  • days of supply compares the current stock in a day to demands/outgoing shipments of the upcoming days
  • incoming deliveries/production are not included in the calculation for a single day
• a unit test for the calculation was provided
  • the test uses the sample data provided in the standard document
  • note: There seems to be a mistake in the standard data. the days of supply for day 5 should be 0

resolves #771

Pre-review checks

Please ensure to do as many of the following checks as possible, before asking for committer review:

• DEPENDENCIES are up-to-date. Dash license tool. Committers can open IP issues for restricted libs.
• Copyright and license header are present on all affected files
• If helm chart has been changed, the chart version has been bumped to either next major, minor or patch level (compared to released chart).

[tom-rm-meyer-ISST]

Thanks a lot for the fast fix! Had smaller findings. Please check.

[tom-rm-meyer-ISST]

LGTM, thanks a lot!