postPredMovements: A function to plot posterior predicitve checks on the model
In tbonne/moveStan: Posterior checks for movement model building

This function uses samples from the stanfit object to generate predictive regions in 2d.

1 2	postPredMovements(model.fit, df.obs, contours = c(25, 50, 75), rangePred = 1:10, numbDraws = 500, buffer = 10)

`model.fit`	stanfit model, containing y_pred and d_pred as generated quantities: y_pred is the model predicted angle, and d_pred is the model predicted step length.
`df.obs`	dataframe of observed travel points.
`contours`	Contours used in the plot of the kernel density of predicted points.
`rangePred`	Range of observed travel points to predict.
`numbDraws`	Number of predictions to make for each observed point.
`buffer`	Spatial buffer for the extent of the output plot around observed points.