add benchmarks

[SimeonEhrig]

• setup infrastructure for catch2 benchmark
• addresses issue #42

TODO

• [x] write documentation
• [x] implement thrust benchmark
• [x] add reduce benchmarks
• [x] implement cmake benchmark custom target (ctest runs the benchmarks in parallel) Use cmake property, which allows to run test serial every time

[SimeonEhrig]

Cool stuff! The most interesting question for me is still: Where will you run these benchmarks and how do you report the numbers?

That's something, which didn't solved yet. The motivation for the benchmarks is the student project. They should provide some benchmarks about it's work. I thought, it is a good idea to integrate the benchmarks in vikunja now and later, if we solved the automatic benchmark problem, we can use it.