Improve testing

[zenoone]

The current regression tests are prone to numerical instabilities. Increasing tolerances and running them on the cpu helped, but does not fully eradicate the problem. A further increase of the tolerances eventually defies the purpose of the tests as the numerical noise can cover up actual changes, leading to a false sense of security. A better solution would be to rework the testing and split the numerically challenging regression tests into smaller unit tests. In this effort the test coverage should be further increased.

[zenoone]

In preparation with the next update of deepqmc.