compatibility of water isotopes with soil resistance

[bandre-ucar]

Summary of Issue:

Water isotopes depends on the soilbeta parameter in several places (CanopyFluxes, BareGroundFluxes). soilbeta is only initialized if soil_resis_method == leepielke1992. It is not used in sl_14. The default in clm5 is sl_14. Either someone needs to implement an sl_14 version of water isotopes without soilbeta, or we need to restrict clm water isotopes to require leepielke1992.

Initially just assigning a dummy value of soilbeta == 1 for water isotopes to get around this.