Interpolates concentrations. Used by correct.xx functions to interpolate concentrations. Uses linear interpolation unless method is 2 (log down), c1 > c2, and both concentrations are non-zero.
interpol(c1 = NA, c2 = NA, t1 = NA, t2 = NA, t3 = NA, method = 1)concentration 1 lagconc
concentration 2 leadconc
time 1 tiem where conc should be calculated
time 2 lagtime
time 3 leadtime
calculation method (1, 2, or 3)