interpol: Interpolate Concentrations
In qpNCA: Noncompartmental Pharmacokinetic Analysis by qPharmetra

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.

1	interpol(c1 = NA, c2 = NA, t1 = NA, t2 = NA, t3 = NA, method = 1)

`c1`	concentration 1 lagconc
`c2`	concentration 2 leadconc
`t1`	time 1 tiem where conc should be calculated
`t2`	time 2 lagtime
`t3`	time 3 leadtime
`method`	calculation method (1, 2, or 3)