Is there any way to specify nested covariance structure?

[bohyunshin]

Hi, first of all, thanks for making this great library.
Wonder if I can specify nested covariance structure or not.
When I specify nested group structure by passing a data frame including two columns to clusters_train object, it raises error that it can only accept Series, which means only one columns.

So far I could not find how I can specify it.
Thanks for reading this issue!