Viewer crashes when the covariance matrix for a confidence ellipse is not PSD

[nicolaje]

When a confidence ellipse is drawn from a covariance matrix that is not PSD, the viewer crashes.
Although the client is responsible for sending a correct covariance matrix, the viewer should not crash when a wrong one is passed.

• Check that the EV of the matrix are positive
• When only one is positive, draw a circle whose radius is function of the only positive EV
• When none are positive, do not draw anything

in the last two situations, an error must be logged.