RooGradMinimizer test and benchmark suites

[egpbos]

For #11, what remains is to do some additional tests. These can also be used when we move on to a parallelized version of RooGradMinimizer, so we should probably build them as real roottest tests and also make rootbench benchmarks.

• 1D (Gaussian)
  • test
  • benchmark
• multi-dimensional (also Gaussian or something else? maybe with an added background)
  • test
  • benchmark
• small "realistic" workspace(s)
  • test
  • benchmark
• big "realistic" workspace(s)
  • test
  • benchmark

[egpbos]

Talking to some ROOT devs, it seems that using Google Test might be a good idea. This would become the second test setup for roofit, since there is already the stressRooFit.cxx framework. Eventually, full migration to gtest might be an option, see #20.

[egpbos]

1D Gaussian test added.

[egpbos]

Added two tests: sum of 5 Gaussians and a branching tree of dependent pdfs, just to see how it performs under odd circumstances (it forces MIGRAD to back out of regions and other oddities).