check.input: Check qpNCA function arguments for validity
In qpNCA: Noncompartmental Pharmacokinetic Analysis by qPharmetra

Checks whether all function arguments are valid and entered column names are present in input data
See qpNCA for description of the arguments

check.input(
  x,
  by = NA,
  nomtimevar = NA,
  timevar = NA,
  depvar = NA,
  bloqvar = NA,
  loqvar = NA,
  loqrule = NA,
  includeCmax = NA,
  exclvar = NA,
  plotdir = NA,
  timelab = NA,
  deplab = NA,
  tau = NA,
  tstart = NA,
  tend = NA,
  teval = NA,
  covariates = NA,
  dose = NA,
  factor = NA,
  reg = NA,
  ss = NA,
  route = NA,
  method = NA
)

`x`	data.frame
`by`	column names in x indicating grouping variables
`nomtimevar`	variable name containing the nominal sampling time after dose
`timevar`	variable name containing the actual sampling time after dose
`depvar`	variable name containing the dependent variable (e.g., concentration)
`bloqvar`	variable name containing the BLOQ flag (0: no, 1: yes)
`loqvar`	variable name containing the LOQ value
`loqrule`	rule number to be applied to the LOQ values in the curve; x$loqrule overrides if provided
`includeCmax`	include Cmax in half-life estimation? (y/n); x$includeCmax overrides if provided
`exclvar`	variable name indicating points to be excluded in half-life estimation (these should have exclvar = 1)
`plotdir`	directory where regression plots (.PNG) will be saved; NA gives default location, NULL skips regression plots
`timelab`	label for time axis in regression plots
`deplab`	label for dependent variable axis in regression plots
`tau`	dosing interval (for multiple dosing); NA (default) if single dose; x$tau overrides
`tstart`	start time of partial AUC (start>0); NA (default) if not requested; x$tstart overrides
`tend`	end time of partial AUC; NA (default) if not requested; x$tend overrides
`teval`	user selected AUC interval (starting at t=0); NA (default) if not requested; x$teval overrides
`covariates`	covariates dataset; Must contain the dose variable
`dose`	variable containing the dose amount
`factor`	conversion factor for CL and V calculation (e.g. dose in mg, conc in ng/mL, factor=1000); x$factor overrides if provided
`reg`	regimen, "SD" or "MD"; x$reg overrides if provided
`ss`	is steady state reached (y/n); x$ss overrides if provided
`route`	route of drug administration ("EV","IVB", "IVI"); x$route overrides if provided
`method`	method for trapezoidal rule; x$method overrides if provided