decouple the testing requirements of ansys-mapdl-core

[jgd10]

The test suite relies on the installation of ansys-mapdl-core to operate properly. However, this also relies on an installation of this package, which results in a circular requirement and you end up installing the latest version of the package into your venv alongside the development version which is a recipe for confusion.

We should decouple these as suggested by Alex in #99

"by caching the expected results to *.npy arrays within testfiles."

[jgd10]

At the moment tests won't run in PyCharm "Out of the box". And if you run pytest itself within the tests folder, it uses the installed version and not the development version.

[jgd10]

So, the solution to this was to install in development mode and use Python 3.8 (3.9 didn't work). I have summarised the steps I took and added them to the README on the project. This was merged in in PR #103. The tests are still coupled, but the original problem is gone. This ticket should remain open, but is of lower priority now.