Change tests to conform with standard notation and newer versions of pytest

[solvejgdinger]

There were several things which needed changing

• A pytest.fixture should not be called direclty but only as argument to a test function. Fixtures can be modular to build more complex structures.

• The scope parameter has to be be defined direclty in the fixture declaration. Instead, as function argument, it get's ignored (scope='function' by default). Also, in most cases a scope='function' is more appropriate.

Interesting to note here is that the number values in the test fucntions change depending on how often the fixtures are called (scope='module' vs 'function'). E.g. test_geometry(geometry) will fail if the scope of fixture geometry was changed. This might indicate that there are potential problems with hidden parameters which do not reset within the pyplis classes.

I tested on python 2.7.13 with pytest 3.6.1 and on python 3.6.8 with pytest 4.0.2. I did not retest in python 3.7. Only failure is ex06 in python 2 as discussed in #11

[johannjacobsohn]

I made the exact changes yesterday, so +1 from me. Unfortunately, the py36 scripts fail for me on this branch ($ tox -escripts-py36), so I'll merge your changes with mine and try to get this working. I'll open another PR later today.

[johannjacobsohn]

@jgliss PR #16 includes these changes. You might as well merge or delete this PR.