ci: Get CI running consistently

[fabern]

ci: Add macOS and Julia 1.7, remove Julia 1.6

As we still have discrepancies between local and CI testing check if it
is related to the architecture and Julia versions.

Nightly version on ubuntu is passing. Local testing happens on macOS and
Julia 1.7.2

[codecov-commenter]

Codecov Report

Merging #42 (54a8850) into develop (0898c4c) will increase coverage by 0.42%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop      #42      +/-   ##
===========================================
+ Coverage    71.29%   71.71%   +0.42%     
===========================================
  Files           16       16              
  Lines         2139     2139              
===========================================
+ Hits          1525     1534       +9     
+ Misses         614      605       -9     

Impacted Files	Coverage Δ
src/func_MSB_functions.jl	51.47% <0.00%> (+0.36%)	⬆️
src/func_DiffEq_definition_p.jl	91.30% <0.00%> (+0.86%)	⬆️
src/module_KPT.jl	61.58% <0.00%> (+1.69%)	⬆️
src/module_WAT.jl	67.80% <0.00%> (+2.05%)	⬆️

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[fabern]

This was merged, although commit f427f69 needed to be reverted. There still seems to be an issue with floating point accuracy of the regression tests which gives reproducible tests on a local development machine. However, for the regression test to pass on the CI it seems that we'd need to hardcode a solution stemming from the CI to compare the current solution.

Options to solve:

• define reference solution for regression tests on the CI
• find out why the floating point accuracy is not given (linked to RNG seed?)
• increase tolerance when comparing two solutions for regression tests. (Note: that this is somewhat against the purpose of the regression test.)

[fabern]

As of v0.9.5 (merging of PR #51) we are now also successfully testing the regression tests on the CI system.

This issue is resolved.