Expand GH CI

[thorstenhater]

As travis.org will be shutting down soon, we will need to expand our CI at Github. Currently, all tests are run, documentation is build, but only on Ubuntu 20.04 LTS.

We are commited to support the following OSs (although MacOS is not really fixed)

• Ubuntu LTS
  • 18.04
  • 20.04
• MacOS
  • 10.15
  • 11.0

And the following compilers on Linux

• GCC 8, 9, 10
• Clang 8, 9, 10, 11

plus the default Apple Clang on Mac.

CMake 3.12+

Python 3.6.9+

Running all combinations is obviously infeasible; we have discussed the following compromise before

• Ubuntu LTS
  • 18.04
  • GCC 8 and 10
  • Clang 8 and 11
  • 20.04
  • GCC 8 and 10
  • Clang 8 and 11
• MacOS
  • 10.15
  • AClang default
  • 11.0
  • AClang default

This results in 10 runs before factoring in CMake and Python. If we were to follow the same procedure ([min, max]) for both, the total would come in at 40 runs. For my taste that is still too much.

My suggestion is to pare this further down to

• Ubuntu LTS
  • 18.04 (collecting the minimum versions)
  • GCC 8
  • Clang 8
  • Python 3.6.9
  • CMake 3.12
  • 20.04 (collecting the current versions)
  • GCC 10
  • Clang 11
  • Python 3.9.1
  • CMake 3.19.4
• MacOS
  • 10.15 (collecting the minimum versions)
  • AClang default
  • Python 3.6.9
  • CMake 3.12
  • 11.0 (collecting the current versions)
  • AClang default
  • Python 3.9.1
  • CMake 3.19.4

for a total of 6 runs. Note that the versions need to be adjusted when new supported releases appear (max) and when we bump the minima (min).

[bcumming]

The proposed configurations strike a good balance between comprehensiveness and a combinatorial explosion of configurations.

Testing both versions of MacOS is important, because the differences between the two are significant.