Tests does not work out of the box

[Thomas Helfer]

This issue is created as part of the review of the JOSS paper (openjournals/joss-reviews#4135).

The tests do not work out of the box. To run them, I had to:

1. install the package
2. update my environment variables (PYTHONPATH in practice)
3. run the tests with make test

This is fairly cumbersome and unsual and incompatible with most common practices and package managers.

[Thomas Helfer]

Note that requiring ngsolve to be correctly installed and accessible (thanks to environment variables like PYTHONPATH and LD_LIBRARY_PATH) is just fine IMHO.

[Paul Stocker]

Fixed with ad9e06a