Return Xarrays in `CLV` RFM Utility Functions?

[ColtAllen]

rfm_summary is the primary data preprocessing step for CLV modeling. It currently returns a pandas dataframe, but recent changes to the clv API are more conducive to Xarrays for parameter broadcasting.

All models should continue to support pandas, but I'm not particularly familiar with xarray and would like to get some more feedback on this, as this could also be beneficial on the mmm side.