Slope function to calculate the tendency of the curve

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Through a simple linear model adjusted to the trajectories, we can infer the tendency of the epidemic curve, to do so we can construct functions that take the posteriors output from the main function nowcasting_inla and adjust the linear model and return the slope of the trajectories, to give a more quantitative tendency inference:

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Two functions needed, slope.estimate.quant, to receive trajectories and fit a linear model on it and return only the slope, and a generate.estimate.slope, which can uses internally the function and from it return a data.frame, with the statistics of this adjust over the trajectories