Post-2.0 cleanups

[bangerth]

I can think of the following things we ought to do:

• Require deal.II 9.0 in CMakeLists.txt
• Document this where appropriate
• Switch the testers to use deal.II 9.0
• Remove #if DEAL_II_VERSION_GTE(9,0) statements.

This means we are now allowed to use C++11 -- yay!

[tjhei]

Does this make sense to do before the hackathon?
Pro:

• we can hand out starter projects to do c++11 related cleanups.
• we don't have to worry about 8.5 and 9.0 compatibility
  Cons:
• Updating the test infrastructure (and test output) is a lot of work
• This forces everybody to install 9.0 on their machines.

[tjhei]

As a compromise: we could require c++11 (enforce in cmakelists.txt) without requiring 9.0 and start using c++11 stuff today! This would give some nice starter projects...

[bangerth]

You mean continue to allow deal.II 8.5, but require C++11 anyway? That sounds fine with me.

[bangerth]

We are tracking the C++11 uses in #2375.

[bangerth]

We agreed to not require deal.II 9.0 at this time, so let's close this.