Set explicit minimum numpy version (1.19.0)

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

This PR adds an explicit minimum dependency version for numpy (and also adds it to the dependency-tests workflow).

closes #555

[danielhuppmann]

fyi @EmiFej

[codecov]

Codecov Report

Merging #556 (88cf87b) into main (e008f62) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #556   +/-   ##
=====================================
  Coverage   93.6%   93.6%           
=====================================
  Files         48      48           
  Lines       5242    5242           
=====================================
  Hits        4909    4909           
  Misses       333     333           

Impacted Files	Coverage Δ
setup.py	0.0% <ø> (ø)

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