computeCovariances returning bool and not clearing

Current implementation is: void computeCovariances(...). If the specified blocks couldn't be computed, the only way to know it is because the covariance matrix blocks stored in Problem have been cleared and now is empty.

In my opinion, this (these) method(s) should return a boolean. And they shouldn't clear the old covariances, if the new ones are not available, at least, keep the old ones...

I have it implemented in gauss branch. If you agree, I could move it to devel. @jsola @mederic_fourmy