Utility for flattening a Dataset

[sethaxen]

Several plots and diagnostics operate only on marginals; as a result, we have a need for a utility function that flattens a Dataset to a single variable with 3dimensions: (chain, draw, var_name). For some variable x with dims (chain, draw, dim1, dim2), dim1=[:a, :b], and dim2=[1, 2], var_name would have dimensions like [x[a,1], x[b, 1], x[a, 2], x[b, 2]].

This utility should have a short name and simple syntax so that it can be used where tables are expected. The return value should be a DimArray. Alternatively, a subset of variables to be flattened could be specified, or variables to be flattened independently could be specified. The latter would facilitate making a corner plot.