Fix for gtest_problem in Core plugin

In the branch plugin/core the test gtest_problem now fails because it requires to install a sensor_camera in the problem to do some testing, but in the core plugin there will be no camera. One option would be to replace this camera with an odom2D sensor or another odom3D sensor.

For more details, the two tests that fail within gtest_problem are:
Problem.StateBlocks
Problem.Covariances