Fixing backwards-compatibility in updatePid() 

This makes the internal computations (like the error term of updatePid() keep the same sign that they did before the API change by bringing back the old updatePid() function contents.

Of course mixing calls to `updatePid()` and `setError()` will mess with the value of the integral gain and the derivative computation (anything that depends on the state), so I also added some warnings about that in the documentation.

This regression was detected by the new tests.