Add shields badges to readme

[maurerle]

New badges can be seen here:

https://github.com/assume-framework/assume/blob/shields-badges/README.md

[maurerle]

We should also add SPDX license headers to our source files and register here:
https://api.reuse.software/register
so that we get a REUSE compliant badge like amiris does:
https://gitlab.com/dlr-ve/esy/amiris/amiris

Being REUSE compliant has non-technical benefits - it allows other researchers to be clear about the terms under which this project can be used and contributed to.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d092ee9) 80.37% compared to head (0ab5f13) 80.37%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #223   +/-   ##
=======================================
  Coverage   80.37%   80.37%           
=======================================
  Files          39       39           
  Lines        4076     4076           
=======================================
  Hits         3276     3276           
  Misses        800      800           

Flag	Coverage Δ
pytest	80.37% <ø> (ø)

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