Feature/downsampling unit tests

[simonlynen]

Added unit tests for:

• Halfsampling
• Twothirdsampling
• Fixed minor bug: 13 instead of 12 here: https://github.com/ethz-asl/ethzasl_brisk/blob/master/brisk/src/image-down-sampling.cc#L701

@sleutenegger This is interesting there was a mini bug in the twothird-sampling but it apparently does not affect the detection. I get the exact same results (kpts and descriptors) after changing it.