get covariance breaks if argument is indexed using sequence not key

If an argument is indexed from the registry using a sequence instead of a key, then `get_covariance()` fails. **_EG_**: If there's a dynamic calculation, then when you need to get the covariance of a different timestep then the registry index will not be a key, but a sequence with a key and an index. But `get_covariance()` just calls `.get(<key>, 0.0)` so this fails.