Stop creating columns that are not needed

[billdenney]

The columns for "volume" and "duration" are always created, but they aren't needed in many cases. Only create them when required, and see if there are others which are automatically created which are not needed. (Note: think more before removing the "exclude" column.)

library(PKNCA)
theoph <- as.data.frame(Theoph)
PKNCAconc(theoph, conc~Time|Subject)
#> Formula for concentration:
#>  conc ~ Time | Subject
#> With 12 subjects defined in the 'Subject' column.
#> Nominal time column is not specified.
#> 
#> First 6 rows of concentration data:
#>  Subject   Wt Dose Time  conc exclude volume duration
#>        1 79.6 4.02 0.00  0.74    <NA>     NA        0
#>        1 79.6 4.02 0.25  2.84    <NA>     NA        0
#>        1 79.6 4.02 0.57  6.57    <NA>     NA        0
#>        1 79.6 4.02 1.12 10.50    <NA>     NA        0
#>        1 79.6 4.02 2.02  9.66    <NA>     NA        0
#>        1 79.6 4.02 3.82  8.58    <NA>     NA        0

^{Created on 2022-05-11 by the reprex package (v2.0.1)}