KalmanFitter uses Acts::Result

This makes use of Acts::Result to explicitly communicate failure states. Based on !679 (merged) so that would have to be merged first.

Discussion point: if an error is returned, the result struct (which contains the track states) will be discarded. This means that it is not possible to recover numbers in response to an error return. Currently, the only thing that triggers such an error is if the smoother runs into the case where a covariance is not invertible. I would argue that in such cases, explicitly failing is important, but I'm not entirely sure if there's a use case where we would need to recover the numerical result. If that would be required, some changes to Acts::Result would be necessary, in principle it should be possible to allow a value AND an error, but I'd only implement this if we think it's necessary. Thoughts? @xai @asalzbur @msmk

Closes #625 (closed)