new_covariate: New covariate
In ronkeizer/PKPDsim: Simulate ODE models

Describe data for a covariate, either fixed or time-variant

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new_covariate(value = NULL, times = NULL, implementation = "interpolate",
  unit = NULL, interpolation_join_limit = 1, remove_negative_times = TRUE,
  comments = NULL, verbose = TRUE)

`value`	a numeric vector
`times`	NULL for time-invariant covariate or a numeric vector specifying the update times for the covariate
`implementation`	for time-varying covariates either 'LOCF' (last observation carried forward) or 'interpolate' (default)
`unit`	specify covariate unit (optional, for documentation purposes only)
`interpolation_join_limit`	for 'interpolate' option, if covariate timepoints are spaced too close together, the ODE solver sometimes chokes. This argument sets a lower limit on the space between timepoints. It will create average values on joint timepoints instead. If undesired set to NULL or 0.
`remove_negative_times`	'TRUE“ or 'FALSE'
`comments`	'NULL', or vector of length equal to 'value' specifying comments to each observation
`verbose`	verbosity