get_traj_velocity: Returns the veclocity of growth trajectories for _all_...
In hafen/hbgd: Healthy birth, growth & development

Returns the veclocity of growth trajectories for _all_ subjects

1 2	get_traj_velocity(all_traj, z_score = TRUE, agedays_floor = 0, agedays_limit = 365, FUN = mean_na_rm_generator())

`all_traj`	Trajectory object returned from `fit_all_trajectories`. Assumes all required data are present.
`z_score`	Boolean indicating whether 'y_var' parameter was for z-score data or not. Default is TRUE.
`agedays_floor`	Numeric, indicating the _floor_ (or start of range) of the number of 'agedays' (inclusive) associated with the derivative of the trajectories, to which the function in 'FUN' argument should apply to. Default is 0. Note, this parameter depends on the the number of grid points used to generate the trajectories, which is specified by the `xg` parameter for the functions `hbgd::fit_trajectory` and `hbgd::fit_all_trajectories`.
`agedays_limit`	Numeric, indicating the _ceiling_ (or end of range) of the number of 'agedays' (inclusive) associated with the derivative of the trajectories, to which the function in 'FUN' argument should apply to. Default is 365. Note, this parameter depends on the the number of grid points used to generate the trajectories, which is specified by the `xg` parameter for the functions `hbgd::fit_trajectory` and `hbgd::fit_all_trajectories`.
`FUN`	A function that will be applied to the first derivative of the fitted growth trajectory. This function should accept a single argument; a vector containing the values of the first derivative. Default value is a _https://en.wikipedia.org/wiki/Currying_ version of the `mean` function, which removes `NA` values by default.