get covariance breaks if argument is indexed using sequence not key

[mikofski]

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.