Use [`vinterp`](https://github.com/mkstratos/stj_pv/blob/092dc88fab708084a3136a1c1c08153ea6157721/STJ_PV/utils.py#L108) function in [isobars2isentropes](https://github.com/lukelbd/climate-benchmarks/tree/master/isobars2isentropes).
