Add end_conc_drop imputation method

[Gero1999]

Adds PKNCA_impute_method_end_conc_drop(), which removes a concentration sitting exactly at time == end for an interval (no-op if none is present).

Because it works through the impute column, it's per-interval: drop the boundary point only on profiles where it belongs to the next dose (e.g. an imputed C0), while leaving genuine troughs untouched.

intervals <- data.frame(
  start  = c(0, 24), end = c(24, 48), tmax = TRUE,
  impute = c("end_conc_drop", NA)   # interval 1 only
)