Fix unittest, clean up and proper disturbance value for NumDiff
In this PR I did the follows:
- Removing any gepetto-viewer or printing message for all unit-tests.
- Defining a proper disturbance value as is described here: #120 (closed).
- Robust asserts that considers the disturbance values (some of them they were also pretty conservative).
- Removing useless module inclusions inside the unit-tests.
With this PR I fixed all little problems as described here #84 (closed). For that, I conducted a stress test for all unit-test (i.e. I run 100 times the test without any failure).