Drop c++03

[MarkusPrim]

Remove c++03 as an option from the CMakeLists and drop the support. This also changes the version number to 2.1.0 to mark the break of backward compatibility.

[MarkusPrim]

@Jojosito Any objections?

[Jojosito]

I have no objections. Just to be on the safe side I would like to wait for an answer from panda. I don't know which c++ version they are using right now.

[ThomasHauth]

Did you already contact someone from Panda about this question or can we add a Panda-Person to this PR?

[MarkusPrim]

This PR is more symbolic, because we broke c++03 compatibility a bit earlier on the master branch. The C++03 compatibility was broken when we lost the boost dependency, because we used the c++11 pointer types to get rid of the boost equivalents (see commit 7d53e80) . When we decide to support c++03, we have to revert this too.

[MarkusPrim]

Another comment: We can keep the ROOT5 compatibility with c++11, see PR #35

[Jojosito]

I am in contact with the Panda software coordinator. He will address the issue in their meeting on monday. They are still using c++03, but are likely to switch.
Anyway, I guess we ca do the transition, and until panda does it as well, they will have to use some older genfit version which is still c++03.

[ThomasHauth]

Thanks for checking back with the Panda colleagues, I will then merge this PR.