Feature/#486 add gcc 7 to ci pipeline

Closes #486

With this merge request gcc 7.3.1 becomes available, which also causes an update from glm 0.9.8.5 to glm 0.9.9.0.

The drawback: glm changed quite a lot, e.g., a GLM_ENABLE_EXPERIMENTAL needs to be set if the glx-stuff is used, there are no default initializations of vector, matrix and quaternion types anymore, and - worse for us right now - there is an potentially uninitialized variable in the glm-code, causing an error in our code if we want to compare quaternions in our tests. Thus, we temporarily use an element-based quaternion comparison untill a new version of glm is available. For links providing more information on the aspects, I mentioned above, I refer the interested reader to our issue #486.