Compiler Warnings in Travis

[Psirus]

This will add two additional jobs to our Travis build that fail when there are any compiler warnings. These have been missed previously, because one usually doesn't test on multiple compilers on ones own machine. Additionally, the flag -Wextra (see #21) has been added. This yields quite a few warnings that should be fixed.

Note:

• These "Compiler warnings" jobs are allowed to fail; even if there is a warning, you'll still get the green tick.
• They fail on the first warning, even if there are more, to reduce Travis time. The corresponding "Normal build" will have all the warnings.
• This is only applied to the PDE branch, and I think it should stay that way for now. Fixing all the warnings in master is not worth the effort imho.

[codecov[bot]]

Codecov Report

Merging #137 into PDE_reviewed will not change coverage. The diff coverage is 70%.

@@              Coverage Diff              @@
##           PDE_reviewed     #137   +/-   ##
=============================================
  Coverage         73.39%   73.39%           
=============================================
  Files               191      191           
  Lines              6442     6442           
=============================================
  Hits               4728     4728           
  Misses             1714     1714

Impacted Files	Coverage Δ
...ics/integrationtypes/IntegrationType3D4NGauss1Ip.h	0% <ø> (ø)	:arrow_up:
src/mechanics/timeIntegration/TimeControl.h	81.81% <0%> (ø)	:arrow_up:
...s/integrationtypes/IntegrationType3D6NGauss1Ip.cpp	0% <0%> (ø)	:arrow_up:
...s/integrationtypes/IntegrationType3D4NGauss1Ip.cpp	0% <0%> (ø)	:arrow_up:
src/mechanics/integrands/NeumannBc.h	100% <100%> (ø)	:arrow_up:
src/mechanics/constitutive/LinearElastic.h	100% <100%> (ø)	:arrow_up:
test/mechanics/cell/CreepLaw.cpp	96.5% <100%> (ø)	:arrow_up:
src/mechanics/timeIntegration/TimeControl.cpp	97.87% <100%> (ø)	:arrow_up:
src/mechanics/dofs/DofMatrixContainer.h	100% <100%> (ø)	:arrow_up:

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[TTitscher]

I cleaned some warnings. The remaining warnings only occur in release mode. Some parameters are only required in assert. How do we deal with that? (That only occurs in old NuTo stuff...) Edit I realized we only do Werror in debug builds. Problem solved...

[Psirus]

Hm, I don't see that as "solved". Rather, should we then not test both versions? And passing parameters just for asserts seems kinda weird.