adding a new flavour of model which is hill_carryover

[shubham223601]

The pull request implements a new variant of the model("hill_carryover") which applies saturation on top of carryover formulation of model. This implementation aligns with the paper
[(Jin, Y., Wang, Y., Sun, Y., Chan, D., & Koehler, J. (2017). Bayesian Methods for Media Mix Modeling with Carryover and Shape Effects. Google Inc.)]

[google-cla]

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