Move CovarianceEstimation support to extension

[andreasnoack]

I noticed that PareteSmooth ended up installing my TSVD package via this one. That was a bit surprising to me so maybe it would make sende to move the CovarianceEstimation definitions here to an extension.

[mcabbott]

This would be better. However, it's a full dep of NamedDims:

https://github.com/invenia/NamedDims.jl/blob/master/Project.toml#L9

so the first step is probably invenia/NamedDims.jl#217